Replace Relish links with GitHub links

[honzajavorek]

Relish is closing, according to #57 (comment). Before we have time to tackle #21, we should remove Relish from everywhere, replace it with links to Gherkin files on GitHub, and close the account.

• replace all references to https://relishapp.com/ in gavel-spec docs (README, etc.)
• replace all references to https://relishapp.com/ in Gavel docs (README, etc.)
• remove publishing scripts from Gavel package.json and who knows where else
• replace all references to https://relishapp.com/ in Dredd docs (both README and Sphinx docs)
• replace all references to https://relishapp.com/ in the core app (code comments, ...)

[honzajavorek]

Also, @Almad I think you don't need to think about tracking our Relish usage anymore 🙂