Stop accidentally defining RSpec

In `lib/time_warp.rb`, `TimeWarpAbility` is monkey-patched onto `RSpec::Core::ExampleGroup`. `RSpec` is the root module of RSpec 2 (in RSpec 1, it is `Spec`). Some other gems rely on this fact to determine whether they are being loaded in the context of RSpec 1 or 2, so defining RSpec here can cause them to believe falsely that they are in the context of RSpec 2, and b0rk.