install-server: Pin rspec (rspec-core) to < 2.99

Pin to rspec < 2.99 until rspec-puppet and serverspec officially
supports rspec 3.x.  This is to avoid warnings due to deprecated
matchers in the rspec-puppet and JUnitFormatter gem.
Merge pull request #59 from enovance/fix-rspec-3.0

install-server: Pin rspec (rspec-core) to < 2.99