Allow http urls for repositories

[corasaurus-hex]

If you use an http url you get this error:

zenflow release review
-----> Looking up pull request for release/ism-pr-provider-independent
/Users/bob/.rvm/gems/ruby-2.2.2@my-gemset/gems/zenflow-0.8.12/lib/zenflow/helpers/pull_request.rb:64:in `[]': no implicit conversion of String into Integer (TypeError)

It took some spelunking to figure out where the issue was, but it's a problem with Zenflow::Repo slug parsing.