[Works OK, but have the travis secure env var problem] Cache gems on s3 for quicker travis CI

- bundle_cache gem depends on nokogiri unfortunately so still has to install that
- shaves about 2 minutes; see https://travis-ci.org/apelade/rag/builds
- **build will fail** because secure env vars on a fork, **but see it work at https://github.com/apelade/rag/pull/8**