Use rake from the current bundle for Rubinius

Rake might not be available on the system, but if there is a Gemfile,
assume that it bundles rake.
Install Rake and Bundler in isolation when missing

Some Rubies need Rake or Bundler for the installation process. However,
since the host Ruby version might not have those gems installed, install
them in a temporary GEM_HOME and add their executables to PATH.

Fixes #426
Fix `unstub` cleanup process for failed stubs
Reduce boilerplate in tests
Add test for automatic fetching of rake gem
Add ability to debug stub calls in testing

Example:

        export RAKE_STUB_DEBUG=2
Merge branch 'provide-bundle-rake'