Ease development, improve testing

This PR eases local development by removing and ignoring `Gemfile.lock` and `.ruby-version`. Let the bundler and the gemspec enforce versions. (Digression: installing ruby 1.9.3-p0 with `ruby-build` on a recent OS X requires installing a version `gcc` without llvm, which seemed like a bridge too far to get marginalia tests running locally).

It also improves test coverage by adding travisci builds for each supported minor version of ActiveRecord/ActionPack (2.3, 3.0, 3.1, 3.2); and ruby 2.0.0.

AFAIK, these are generally accepted best-practices for gems; but let me know if not.

(If I'm super lucky, all these new travisci builds will pass on the first try...but it'll probably take some mucking with.)