Peg version of rspec in gemspec

The test suite fails under rspec 3. To prevent others from running into this problem, we can pessimistically peg the version to the 2.14.x line. Note that the test suite passes on 2.99.x, but gives a ton of deprecation warnings.

I know it's generally not best practice to specify dependencies in gemspecs because users may have unforeseen combinations of gems installed, but this seems pretty reasonable since it makes beginning development on the centurion gem a bit less mystifying.