Remove tilt warning

This commit would remove the following warning in Travis
WARN: tilt autoloading 'coffee_script' in a non thread-safe way; explicit require 'coffee_script' suggested.
Revert "Remove tilt warning"

This reverts commit e88b0507011cf2cda4beadf27ff53f9a9b885b9e.
Remove tilt warning

Try another way again!!
Merge pull request #59 from kaznum/remove_tilt_error

Remove tilt warning