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.
Ok, it’s not that simple: We need to know if we‘re running RSpec 2 or 3 to support the different syntaxes for running a specific example (RSpec 2 uses `--line 123`, RSpec 3 needs `path/to/spec.rb:123`). So I currently don’t see how we could get rid of `rspec3?` …  :confused: