Hmm, I'm on the fence about that.  it's really only mocks and expectations that will need diff-lcs (since core won't be doing any diffs directly) but the logic is here now.    The only downside to putting it here I can think of is for when people use rspec-core on its own w/o rspec-expectations and rspec-mocks -- in that case, `diff-lcs` won't be needed but will be resolved with their bundle and installed.

I think few users do that so having it here seems OK to me.