Add Travis config

Test against the following Ruby versions:

- 2.0.0
- 1.9.3
- 1.9.2
- jruby-19mode (1.6.7 in Ruby 1.9 mode)
- rbx-19mode (in Ruby 1.9 mode)
- ruby-head (upgraded every 2-3 weeks)
- jruby-head (upgraded every 2-3 weeks)