.travis.yml: Add compilation with gcc 5 and gcc 6

Now that we have a way to correctly set a matching coverage tool, we can
add more recent compiler versions to the Travis build.
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

.travis.yml

@@ -5,6 +5,22 @@ matrix:
   include:
   - compiler: gcc
     env: CFLAGS="-std=gnu99"
+  - compiler: gcc-5
+    env: GCOV="gcov-5"
+    addons:
+      apt:
+        sources:
+          - ubuntu-toolchain-r-test
+        packages:
+          - gcc-5
+  - compiler: gcc-6
+    env: GCOV="gcov-6"
+    addons:
+      apt:
+        sources:
+          - ubuntu-toolchain-r-test
+        packages:
+          - gcc-6
   - compiler: clang
     addons:
       apt:
@@ -17,6 +33,26 @@ matrix:
           - valgrind
     env: CFLAGS="-std=gnu99"
     compiler: gcc
+  - dist: trusty
+    compiler: gcc-5
+    env: GCOV="gcov-5"
+    addons:
+      apt:
+        sources:
+          - ubuntu-toolchain-r-test
+        packages:
+          - gcc-5
+          - valgrind
+  - dist: trusty
+    compiler: gcc-6
+    env: GCOV="gcov-6"
+    addons:
+      apt:
+        sources:
+          - ubuntu-toolchain-r-test
+        packages:
+          - gcc-6
+          - valgrind
 
 script:
     - make -j2 -k quiet=1