Remove support for rspec1

See #78. Not sure if this is 100% correct/complete, so I’ll let this PR sit around for some days to have more time to think about it.
Currently `rspec3?` is used in [`Runner#run`](https://github.com/rspec/rspec-tmbundle/blob/master/Support/lib/rspec/mate/runner.rb#L41) to check what formatter to use (until RSpec 2 the TextMateFormatter was included in RSpec, for RSpec 3 we need to require it from the bundle). But now that I think of it: Maybe it’s better to just check if `RSpec::Core::Formatters::TextMateFormatter` is defined? I’ll give it a try.

Thanks for helping me in thinking this through!