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.

Removed:
- Rubinius (rbx) as it does not ship with psych as part of the standard
  build.