Support rspec 3.0

Base idea is same as #8.

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

Thank you.
Perhaps a little future-proofing here?

~~~ ruby
case ::RSpec::Core::Version::STRING.to_i
when 2 then require_relative 'rspec2'
when 3 then require_relative 'rspec3'
else
  fail 'requires rspec version 2 or 3'
end
~~~