Clean up gem dependencies (fix #1148)

* Remove unused development dependency on open4
* Use pessimistic versioning for coderay, rake, and simplecov
* Bump versions of rake and mocha
* Loosen pessimistic versioning on mocha and bond

The only remaining warning from RubyGems is on coderay -- we chose not to use
'~> 1.1' because there were breaking changes between 1.0 and 1.1.