Bump RSpec and add Mocha

Only include the bits of RSpec that we need, and
add Mocha for stubbing.
Add a spec_helper to tidy things up

Tidy up the tests and make sure that they're requiring
files from the local directory and not the installed
gem.
Allow a different Foreman executable to be specified

If the BOWLER_FOREMAN_EXEC env variable is present,
use this instead of foreman to start the processes.

(eg. "forego")
Merge pull request #5 from JordanHatch/add-support-for-forego

Allow an alternative foreman executable to be specified