refresh travis setup

- indicate that we're keen on caching
- lets not install non-essential gems
- test against a larger variety of ruby interpreters however, lets allow some failures
- enable speedy greens (http://blog.travis-ci.com/2013-11-27-fast-finishing-builds)
temporarily ensure test dependencies are installed #88 should fix this
constrain to 1.8.7 compatible version of Rake

- http://docs.seattlerb.org/rake/History_rdoc.html#label-10.1.2+/+2014-03-24
Merge pull request #86 from karlfreeman/travis-refresh

Refresh Travis setup