Specify rake and rspec versions in gemspec

Since the development dependency versions are undefined, gem just grabs
the latest of everything. But the specs are failing under RSpec 3.0.0.
Specify Rubinius version

The build is currently failing because no binary package is available
for 'rbx'. According to The Internet, specifying the version might help.
And that seems like a good idea, anyway.
Merge pull request #1 from adammck/lock_dependency_versions

Lock dependency versions