Remove mocha dependency and rspec pinning

Since mocha isn't really used any more, if a module needs mocha then it
should require mocha.

Since modules are written with rspec 2 or 3 syntax, PSH should not
declare dependencies on rspec at all, but leave it up to the module to
declare the dependency.
Merge pull request #59 from hunner/remove_mocha

Remove mocha dependency and rspec pinning