Enabling Travis
Can't hardcode the Ruby version

Maybe we want a matrix of builds, in which case hardcoding the Ruby
version doesn't make much sense. :)
Constraining Ruby version and enabling Redis

The ohai dependency requires Ruby 2.0 or higher, so no jruby or rbx
support. Lita doesn't support those anyway.
Merge pull request #3 from PagerDuty/enable_travis

Enabling Travis