travis: restrict build matrix further

We don't actually want to test all possible combinations; we just want
to make sure that each thing (e.g. linux/osx, ffmpeg/libav) is tested
once.

Exclude Linux + ffmpeg-stable, because ffmpeg-stable is already tested
on OSX.

Exclude clang on Linux, because OSX needs clang, but Coverity (running
on Linux) needs gcc - so we use gcc only on Linux.

I also wanted to reduce the matrix to a single configuration when
running Coverity, but apparently this is not possible.
(See travis-ci/travis-ci#1975.)

.travis.yml

@@ -32,6 +32,10 @@ matrix:
       env: LIBAV=ffmpeg-git
     - os: osx
       compiler: gcc
+    - os: linux
+      env: LIBAV=ffmpeg-stable
+    - os: linux
+      compiler: clang
 
 before_install: ./travis-deps libass-stable $LIBAV
 script: