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

I think it'd make sense to store a file for each tagged release as well (e.g. `batman-0.14.1.tar.gz`). Should we make one for historic versions, or just do that starting for future ones?

@nickjs @angelini @TheJeff