clean environment requires specifying more dependency versions

I tried to build leap_cli locally and ran into a version conflict about net-ssh ~> 2.2.2 (vagrant) vs. net-ssh >= 2.6.5 (capistrano dependencies net-scp, net-sftp and net-ssh-gateway).
bundle resolves these automagically. But if we want to do without bundle we need to specify the exact versions in the gemspec.

I think this is a bad practice and bundler should be used instead. But if bundle has never been run and matching dependencies are not installed anyway we need these.