Remove puppet as a gem dependency

Puppet is used as a runtime dependency of puppetlabs_spec_helper, but
is often installed by system packages and installing the gem could
create conflicts when libraries are loaded without bundler by the system
puppet.

rspec-puppet and puppet-lint do not declare puppet as a dependency, and
0.4.1 of psh did not declare it as a dependency, so it shouldn't now.
Merge pull request #62 from hunner/remove_dep

Remove puppet as a gem dependency