[TASK] Raise RVM and Gem minimum versions

- Add Ruby 2.1 to .travis.yml.
- Drop Ruby < 1.9.3 (all end-of-life versions) from .travis.yml.
- Make 1.9.3 the minimum required Ruby version.
- Make Rails 3.2.0 the minimum required version.
- Change the development requirement to Rake 10.3.2 (the current version).
- Drop any mentions of Rails 2.x from the README.