travis: add gcc to the build matrix

For the purpose of running Coverity correctly.

Although I'm not sure how well this works. gcc won't work on OSX, and
also I'm not sure if Coverity will act up if the build matrix has more
than 1 configuration (will it submit multiple scans?).

.travis.yml

@@ -10,9 +10,11 @@ env:
     - LIBAV=ffmpeg-stable
     - LIBAV=ffmpeg-git
   global:
+    # Coverity token
     - secure: "H21mSRlMhk4BKS0xHZvCFGJxteCP0hRVUxTuNfM2Z9HBsyutuLEYMtViLO86VtM+Tqla3xXPzUdS4ozLwI72Ax/5ZUDXACROj73yW6QhFB5D6rLut12+FjqC7M33Qv2hl0xwgNBmR5dsm1ToP37+Wn+ecJQNvN8fkTXF+HVzOEw="
 compiler:
   - clang
+  - gcc
 
 branches:
   only:
@@ -28,6 +30,8 @@ matrix:
       env: LIBAV=libav-git
     - os: osx
       env: LIBAV=ffmpeg-git
+    - os: osx
+      compiler: gcc
 
 before_install: ./travis-deps libass-stable $LIBAV
 script: