add custom bucket for binary installs

This will allow fetching binary installs from our S3 bucket "travis-rubies". This bucket will be used in addition to the binary installs rvm is provided.

A post request to http://rubies.travis-ci.org/build/ruby-head with a valid signature for ruby/ruby will trigger a new build. Builds run as a job on Travis CI. After compilation, the job will try installing bundler and then running bundle install on a simple Sinatra project. Only if that succeeds will it publish a new binary install. The idea is to add this as a web hook to ruby/ruby. The same thing works for jruby/jruby. It also accepts the signature from rubinius/rubinius, though I haven't got that to build yet. In addition, the bucket currently provides ree-1.8.7-2011.12, as a customer asked for it. Adding more versions should not be an issue.

This has been tested on staging.