Clarifying the role of the Gemfile.

- It's a Travis CI thing. I don't use it otherwise. There is no value in
  it if you're developing a gem.
Changing Travis testing platforms.

Added:
- Ruby 2.1
- ruby-head, and jruby-head.

Changed:
- Rubinius (rbx) to remove mode installs.
- Added configuration to .travis.yml to install Rubinius deps that
  should be included with Rubinius by default.
Merge pull request #50 from halostatue/update-travis

Update Travis platforms.