travis: Exclude gcc for the macOS target

On a normal macOS setup, 'gcc' is a symlink to clang. Therefore there is
little point in running the tests twice, with CC=gcc and CC=clang.

.travis.yml

@@ -11,6 +11,10 @@ addons:
 compiler:
   - clang
   - gcc
+matrix:
+    exclude:
+        - os: osx
+          compiler: gcc
 cache:
   directories:
     - libav-samples