.travis.yml: Add builds under Ubuntu Trusty

At the moment our Travis builds all use Travis's default Ubuntu
Precise base distro.  For wider testing, add a build using their
Ubuntu Trusty distro.  Only build with gcc there, for now, since clang
will cause ccanlint failures, due to the gcov version there not being
suitable for clang output.
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

.travis.yml

@@ -6,6 +6,9 @@ matrix:
   - compiler: gcc
     env: CFLAGS="-std=gnu99"
   - compiler: clang
+  - dist: trusty
+    env: CFLAGS="-std=gnu99"
+    compiler: gcc
 
 script:
     - make -j2 -k quiet=1