CI: Clang 6.0 is not available

But Clang 3.3 and 3.4 are.

* .travis.yml (addons): Remove, it appears to be ignore if the matrix
also defines it.
(matrix): Update.

.travis.yml

@@ -1,13 +1,18 @@
 language: cpp
 
-addons:
-  apt:
-    packages:
-      - ccache
-
 matrix:
   include:
-    # works on Precise and Trusty
+    # GCC.
+    - os: linux
+      addons:
+        apt:
+          sources:
+            - ubuntu-toolchain-r-test
+          packages:
+            - g++-4.6
+      env:
+         - MATRIX_EVAL="CC=gcc-4.6 && CXX=g++-4.6"
+
     - os: linux
       addons:
         apt:
@@ -18,7 +23,6 @@ matrix:
       env:
          - MATRIX_EVAL="CC=gcc-4.9 && CXX=g++-4.9"
 
-    # works on Precise and Trusty
     - os: linux
       addons:
         apt:
@@ -29,7 +33,6 @@ matrix:
       env:
          - MATRIX_EVAL="CC=gcc-5 && CXX=g++-5"
 
-    # works on Precise and Trusty
     - os: linux
       addons:
         apt:
@@ -40,7 +43,6 @@ matrix:
       env:
         - MATRIX_EVAL="CC=gcc-6 && CXX=g++-6"
 
-    # works on Precise and Trusty
     - os: linux
       addons:
         apt:
@@ -51,7 +53,6 @@ matrix:
       env:
         - MATRIX_EVAL="CC=gcc-7 && CXX=g++-7 && CPPFLAGS=-DNDEBUG && CFLAGS=-O3 && CXXFLAGS=-O3"
 
-    # works on Precise and Trusty
     - os: linux
       addons:
         apt:
@@ -62,19 +63,36 @@ matrix:
       env:
         - MATRIX_EVAL="CC=gcc-8 && CXX=g++-8"
 
-    # works on Precise and Trusty
+    # Clang.
+    - os: linux
+      addons:
+        apt:
+          packages:
+            - clang-3.3
+      env:
+        # No versioned name installed.
+        - MATRIX_EVAL="CC=clang && CXX=clang++"
+
+    - os: linux
+      addons:
+        apt:
+          packages:
+            - clang-3.4
+      env:
+        # No versioned name installed.
+        - MATRIX_EVAL="CC=clang && CXX=clang++"
+
     - os: linux
       addons:
         apt:
           sources:
             - ubuntu-toolchain-r-test
-            - llvm-toolchain-precise-3.4
+            - llvm-toolchain-precise-3.5
           packages:
-            - clang-3.4
+            - clang-3.5
       env:
-        - MATRIX_EVAL="CC=clang-3.4 && CXX=clang++-3.4"
+        - MATRIX_EVAL="CC=clang-3.5 && CXX=clang++-3.5"
 
-    # works on Precise and Trusty
     - os: linux
       addons:
         apt:
@@ -86,7 +104,6 @@ matrix:
       env:
         - MATRIX_EVAL="CC=clang-3.6 && CXX=clang++-3.6"
 
-    # works on Precise and Trusty
     - os: linux
       addons:
         apt:
@@ -98,7 +115,6 @@ matrix:
       env:
         - MATRIX_EVAL="CC=clang-3.7 && CXX=clang++-3.7"
 
-    # works on Precise and Trusty
     - os: linux
       addons:
         apt:
@@ -110,7 +126,6 @@ matrix:
       env:
         - MATRIX_EVAL="CC=clang-3.8 && CXX=clang++-3.8"
 
-    # works on Trusty
     - os: linux
       addons:
         apt:
@@ -121,7 +136,6 @@ matrix:
       env:
         - MATRIX_EVAL="CC=clang-3.9 && CXX=clang++-3.9"
 
-    # works on Trusty
     - os: linux
       addons:
         apt:
@@ -132,7 +146,6 @@ matrix:
       env:
         - MATRIX_EVAL="CC=clang-4.0 && CXX=clang++-4.0"
 
-    # works on Trusty
     - os: linux
       addons:
         apt:
@@ -143,17 +156,6 @@ matrix:
       env:
         - MATRIX_EVAL="CC=clang-5.0 && CXX=clang++-5.0 && CPPFLAGS=-DNDEBUG && CFLAGS=-O3 && CXXFLAGS=-O3"
 
-    # works on Trusty
-    - os: linux
-      addons:
-        apt:
-          sources:
-            - llvm-toolchain-trusty-6.0
-          packages:
-            - clang-6.0
-      env:
-        - MATRIX_EVAL="CC=clang-6.0 && CXX=clang++-6.0"
-
 before_install:
   - sudo apt-get update -qq
   - sudo apt-get install -qq autoconf automake autopoint flex gettext graphviz help2man m4 texinfo valgrind