travis-ci: update Libav release

Libav 10 was released, so we can enable testing the stable Libav version
again.

FFmpeg 2.2 was also released, but since we still support 2.1.4, we stick
with the older version. This is better for testing.

.travis.yml

@@ -1,6 +1,7 @@
 language: c
 
 env:
+  - LIBAV=libav-stable
   - LIBAV=libav-git
   - LIBAV=ffmpeg-stable
   - LIBAV=ffmpeg-git

travis-deps

@@ -54,7 +54,7 @@ class Libav < TravisDepsBuilder
     {
       "libav-stable" => {
         :action => :stable,
-        :url    => 'http://libav.org/releases/libav-9.10.tar.gz'
+        :url    => 'http://libav.org/releases/libav-10.tar.gz'
       },
       "libav-git" => {
         :action => :git,