travis: use clang for ffmpeg/libav compilation

I overlooked the fact that the ffmpeg/libav build system only supports `--cc`
and completly ignores $CC. Hopefully this makes the build times a little
faster.

Fixes #332

travis-deps

@@ -72,7 +72,7 @@ class Libav < TravisDepsBuilder
   end
 
   def configure
-    [super, "--disable-doc"].join(" ")
+    [super, "--disable-doc", "--cc=#{ENV['CC']}"].join(" ")
   end
 end