Use the correct version constraint for the RSpec dependency

The current release of RSpec is v3.x. It is incompatible with the specs
defined in amq-protocol due to the elimination of such constructs as
'its' blocks.
Merge pull request #44 from njonsson/correct-rspec-dependency

Use the correct version constraint for the RSpec dependency
Use the correct platform constraint for the ruby-prof dependency

The ruby-prof gem requires Ruby v1.9.3 or newer.