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.
Extract shared example

This is to be able to run the specs on different namespaces
Adds the ability to pass a Proc as namespace
Add examples for changing namespace with Proc
Add some tests for Redis::Namespace#namespace
Fix specs for ruby 1.8.7
Merge remote-tracking branch 'adammck/lock_dependency_versions' into lambda-namespaces