Test against latest Ruby 2.1 release
Configure Travis CI as recommended

See https://github.com/thoughtbot/appraisal#travis-ci-integration
Update appraisal dependency to ~> 1.0
Update rake dependency to ~> 10.3
Update shoulda dependency to ~> 3.5
Update rspec dependency to ~> 2.99
Update phantomjs dependency to ~> 1.9