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.
Merge pull request #25 from ted27/peg_rspec_version

Peg version of rspec in gemspec