Use ruby 2.0.0 in Gemfile and Travis config.

Also, update to latest version of dependencies.
Fix rspec deprecations (stub -> double).
Merge pull request #45 from rjackson/use_ruby_2_0

Use ruby 2.0.0 in Gemfile and Travis config.
Remove RedditJoke plugin.

The jokes that are returned are often inappropriate.

Resolves #38.