Support rspec 3.0

Base idea is same as #8.

@tmm1 When you've done for minitest5, could you review this PR?

Thank you.
While this adds support for 3.x, it'd be wise to ensure we don't assume support for the eventual 4.x:
~~~ diff
- s.add_development_dependency 'rspec', '~> 2.13'
+ s.add_development_dependency 'rspec', '>= 2.13', '< 4.0'
~~~