Require bundled rake version

`require 'rake/testtask'` uses the latest rake version. Zeus fails to start if this version does not match the bundled rake version, i.e. the one specified in Gemfile.lock:

> You have already activated rake 10.0.4, but your Gemfile requires rake 10.0.3.

This fix explicitly requires the rake version as specified in Gemfile.lock.