[ci] downgrade to firefox 11 on travis

firefox 17 caused a regression with selenium-webdriver, random
clicks won't happen and just cause timeouts. This is reported
on selenium's bug tracker, but won't help our builds ;(

dist/ci/obs_testsuite_travis_before.sh

@@ -28,7 +28,8 @@ sudo apt-get install liblzma-dev librpm-dev libxml-parser-perl libfile-sync-perl
 
 case "$SUBTEST" in
  webui*)
-  sudo apt-get install firefox
+  sudo apt-cache show firefox 
+  sudo apt-get install --force-yes firefox=11.0+build1-0ubuntu4
   pushd src/webui-testsuite
   if test "$REMOVEGEMLOCK" = true; then
     rm Gemfile.lock