store build cache
$ bundle clean
0.00s1.26schanges detected, packing new archive
FAILED: tar -Pzcf /home/travis/.casher/push.tgz 
tar: Cowardly refusing to create an empty archive
Try `tar --help' or `tar --usage' for more information.
uploading archive
failed to upload cache
curl: Can't open '/home/travis/.casher/push.tgz'!
curl: try 'curl --help' or 'curl --manual' for more information

Bundler::GemspecError: Could not read gem at /home/travis/build/scala/scala-lang/bundle-vendor/bundle/ruby/1.9.1/cache/highline-1.6.21.gem. It may be corrupted.
An error occurred while installing highline (1.6.21), and Bundler cannot
continue.
Make sure that `gem install highline -v '1.6.21'` succeeds before bundling.