CI: also use GCC 4.7 and 4.8

* .travis.yml (matrix): here.

.travis.yml

@@ -65,6 +65,28 @@ matrix:
       env:
          - MATRIX_EVAL="CC=gcc-4.9 && CXX=g++-4.9"
 
+    - name: "GCC 4.8"
+      os: linux
+      addons:
+        apt:
+          sources:
+            - ubuntu-toolchain-r-test
+          packages:
+            - g++-4.8
+      env:
+         - MATRIX_EVAL="CC=gcc-4.8 && CXX=g++-4.8"
+
+    - name: "GCC 4.7"
+      os: linux
+      addons:
+        apt:
+          sources:
+            - ubuntu-toolchain-r-test
+          packages:
+            - g++-4.7
+      env:
+         - MATRIX_EVAL="CC=gcc-4.7 && CXX=g++-4.7"
+
     - name: "GCC 4.6"
       os: linux
       addons: