chore(travis): temporarily disable Firefox

I can't get it allocated on SauceLabs and so this is failing all the builds, making our Travis build useless.

I contacted folks from SL. Once we figure it out, I will revert this change.

Author: vojtajina

travis_build.sh

@@ -5,5 +5,5 @@ set -e
 export SAUCE_ACCESS_KEY=`echo $SAUCE_ACCESS_KEY | rev`
 
 grunt parallel:travis --reporters dots \
-  --browsers SL_Chrome,SL_Firefox,SL_Safari,SL_IE_8,SL_IE_9,SL_IE_10 \
+  --browsers SL_Chrome,SL_Safari,SL_IE_8,SL_IE_9,SL_IE_10 \
   --e2e-browsers SL_Chrome