Add Ruby dependency information via Gemfiles.

Add Gemfile and Gemfile.lock, both of which are used by Bundler (a
Ruby gem) to encode Ruby dependencies for projects.

- Gemfile.lock is machine-generated (but human-readable) and encodes
  the most specific Ruby dependency information.
- Gemfile is human-written, and typically encodes less specific Ruby
  dependency information. It is used by Bundler if Gemfile.lock does
  not exist. Rubyists recommend committing both files.

Addresses issue #322.