spec: Gemfile - pin gems to rspec < 2.99

Pin to rspec < 2.99 until rspec-puppet officially supports rspec 3.x.
This is to avoid warnings due to deprecated matchers in the rspec-puppet
gem.

See: https://launchpad.net/bugs/1326034
Merge pull request #52 from enovance/pin-rspec

spec: Gemfile - pin gems to rspec < 2.99