Upload the latest build to S3 when master passes travis

This enables `travis-artifacts` for master, uploading the files in `build/dist` to http://batman.js.s3.amazonaws.com/batman-master.tar.gz

This is based on the discussion here: http://irclogger.com/.batmanjs/2013-11-08#1383928218-1383930561

@nickjs @angelini @TheJeff