Revert "Depend on RSpec-expectations, not RSpec"

This reverts commit 8fdd8d2b3c725a2156690458ac7dbc36d5cdd94a.

Cucumber integration relies on RSpec-core.  Possibly it could be
reworked but until then RSpec dependency is back.
Merge pull request #68 from skalee/rspec-3

Bring back full RSpec runtime dependency