Setup Travis CI

Reasons for excluding various Ruby versions:

- 1.8.7: binding_of_caller dependency's native extension fails to build.
- 1.9.2: Capybara requires Ruby 1.9.3+.
- JRuby: C extensions are not supported on Travis, but we need them.
- Rubinius 1.8: Nokogiri requires Ruby 1.9.2+.

Of course, some of the issues with later versions apply to the earlier
versions as well.
Add Travis CI badge to readme