travis: drop libav-stable support

For libav-stable, we download the Libav tarball, which is failing,
because their certificate is broken:

ERROR: cannot verify libav.org's certificate, issued by `/C=US/O=Let\'s Encrypt/CN=Let\'s Encrypt Authority X3':
  Issued certificate has expired.

I don't intend to support Libav's overly old releases anymore anyway,
so if you want to use Libav, use its git master.

.travis.yml

@@ -5,7 +5,6 @@ os:
   #- osx
 env:
   matrix:
-    - LIBAV=libav-stable
     - LIBAV=libav-git
     - LIBAV=ffmpeg-stable
     - LIBAV=ffmpeg-git
@@ -26,8 +25,6 @@ branches:
 
 matrix:
   exclude:
-    - os: osx
-      env: LIBAV=libav-stable
     - os: osx
       env: LIBAV=libav-git
     - os: osx

TOOLS/travis-deps

@@ -87,10 +87,6 @@ end
 class Libav < TravisDepsBuilder
   def build_map
     {
-      "libav-stable" => {
-        :action => :stable,
-        :url    => 'http://libav.org/releases/libav-12.tar.gz'
-      },
       "libav-git" => {
         :action => :git,
         :url    => "git://git.libav.org/libav.git"