Try apt-get update to find llvm-3.6.

Author: brixen

.travis.yml

@@ -8,7 +8,7 @@ before_install:
   - echo $LANG
   - echo $LC_ALL
   - if [ $TRAVIS_OS_NAME == linux ]; then wget -O - http://llvm.org/apt/llvm-snapshot.gpg.key|sudo apt-key add -; fi
-  - if [ $TRAVIS_OS_NAME == linux ]; then sudo apt-get install libllvm3.6 llvm-3.6-dev; fi
+  - if [ $TRAVIS_OS_NAME == linux ]; then sudo apt-get update && sudo apt-get install libllvm3.6 llvm-3.6-dev; fi
   - if [ $TRAVIS_OS_NAME == osx ]; then brew update && brew install llvm && brew link --force llvm; fi
   - rvm use $RVM --install --binary --fuzzy
   - gem update --system