Don't warn about overriding the :install method if we're a GithubGistFormula

A few formulas, "knife-completion" in particular, use the GithubGistFormula subclass of Formula. 

However, warning them about using the regular Formula method, makes them have to reinvent the wheel for using a Github Gist. 

It makes sense to warn for a script file install, where it's just downloading a tarball and you can easily alter the formula. For a Github Gist though, it takes a bit more work, and is a duplication of already existing code.