CI: formatting changes

* .travis.yml: Use the single line form of lists, when reduced to a
singletons.

.travis.yml

@@ -169,8 +169,7 @@ jobs:
       dist: bionic
       addons:
         apt:
-          packages:
+          packages: g++-7
-            - g++-7
       env:
         - MATRIX_EVAL="CC=gcc-7 && CXX=g++-7"
 
@@ -180,10 +179,8 @@ jobs:
       dist: xenial
       addons:
         apt:
-          sources:
+          sources: ubuntu-toolchain-r-test
-            - ubuntu-toolchain-r-test
+          packages: g++-6
-          packages:
-            - g++-6
       env:
         - MATRIX_EVAL="CC=gcc-6 && CXX=g++-6"
 
@@ -193,10 +190,8 @@ jobs:
       dist: xenial
       addons:
         apt:
-          sources:
+          sources: ubuntu-toolchain-r-test
-            - ubuntu-toolchain-r-test
+          packages: g++-5
-          packages:
-            - g++-5
       env:
          - MATRIX_EVAL="CC=gcc-5 && CXX=g++-5"
 
@@ -206,10 +201,8 @@ jobs:
       dist: xenial
       addons:
         apt:
-          sources:
+          sources: ubuntu-toolchain-r-test
-            - ubuntu-toolchain-r-test
+          packages: g++-4.9
-          packages:
-            - g++-4.9
       env:
          - MATRIX_EVAL="CC=gcc-4.9 && CXX=g++-4.9"
 
@@ -219,10 +212,8 @@ jobs:
       dist: xenial
       addons:
         apt:
-          sources:
+          sources: ubuntu-toolchain-r-test
-            - ubuntu-toolchain-r-test
+          packages: g++-4.8
-          packages:
-            - g++-4.8
       env:
          - MATRIX_EVAL="CC=gcc-4.8 && CXX=g++-4.8"
 
@@ -232,10 +223,8 @@ jobs:
       dist: xenial
       addons:
         apt:
-          sources:
+          sources: ubuntu-toolchain-r-test
-            - ubuntu-toolchain-r-test
+          packages: g++-4.7
-          packages:
-            - g++-4.7
       env:
          - MATRIX_EVAL="CC=gcc-4.7 && CXX=g++-4.7"
 
@@ -245,10 +234,8 @@ jobs:
       dist: xenial
       addons:
         apt:
-          sources:
+          sources: ubuntu-toolchain-r-test
-            - ubuntu-toolchain-r-test
+          packages: g++-4.6
-          packages:
-            - g++-4.6
       env:
          - MATRIX_EVAL="CC=gcc-4.6 && CXX=g++-4.6"
 
@@ -304,8 +291,7 @@ jobs:
       dist: xenial
       addons:
         apt:
-          sources:
+          sources: llvm-toolchain-xenial-5.0
-            - llvm-toolchain-xenial-5.0
           packages:
             - clang-5.0
             - libc++-dev
@@ -318,10 +304,8 @@ jobs:
       dist: xenial
       addons:
         apt:
-          sources:
+          sources: llvm-toolchain-xenial-4.0
-            - llvm-toolchain-xenial-4.0
+          packages: clang-4.0
-          packages:
-            - clang-4.0
       env:
         - MATRIX_EVAL="CC=clang-4.0 && CXX=clang++-4.0"
 
@@ -331,10 +315,8 @@ jobs:
       dist: xenial
       addons:
         apt:
-          sources:
+          sources: llvm-toolchain-xenial-3.9
-            - llvm-toolchain-xenial-3.9
+          packages: clang-3.9
-          packages:
-            - clang-3.9
       env:
         - MATRIX_EVAL="CC=clang-3.9 && CXX=clang++-3.9"
 
@@ -347,8 +329,7 @@ jobs:
           sources:
             - ubuntu-toolchain-r-test
             - llvm-toolchain-precise-3.8
-          packages:
+          packages: clang-3.8
-            - clang-3.8
       env:
         - MATRIX_EVAL="CC=clang-3.8 && CXX=clang++-3.8"
 
@@ -361,8 +342,7 @@ jobs:
           sources:
             - ubuntu-toolchain-r-test
             - llvm-toolchain-precise-3.7
-          packages:
+          packages: clang-3.7
-            - clang-3.7
       env:
         - MATRIX_EVAL="CC=clang-3.7 && CXX=clang++-3.7"
 
@@ -375,8 +355,7 @@ jobs:
           sources:
             - ubuntu-toolchain-r-test
             - llvm-toolchain-precise-3.6
-          packages:
+          packages: clang-3.6
-            - clang-3.6
       env:
         - MATRIX_EVAL="CC=clang-3.6 && CXX=clang++-3.6"
 
@@ -389,8 +368,7 @@ jobs:
           sources:
             - ubuntu-toolchain-r-test
             - llvm-toolchain-precise-3.5
-          packages:
+          packages: clang-3.5
-            - clang-3.5
       env:
         - MATRIX_EVAL="CC=clang-3.5 && CXX=clang++-3.5"
 
@@ -401,8 +379,7 @@ jobs:
       dist: trusty
       addons:
         apt:
-          packages:
+          packages: clang-3.4
-            - clang-3.4
       env:
         # No versioned name installed, but beware that Travis installs
         # a more modern clang earlier in the default PATH.
@@ -415,8 +392,7 @@ jobs:
       dist: trusty
       addons:
         apt:
-          packages:
+          packages: clang-3.3
-            - clang-3.3
       env:
         # See comment for 3.4.
         - MATRIX_EVAL='CC=/usr/bin/clang && CXX=/usr/bin/clang++'