Don't create listener until it's needed

Code that's using this gem as a library dependency may just want to take advantage of its rendering, not its watching, so defer creating a listener until it's needed.

This pull also updates deps and such to ensure tests are green, which includes [pinning the optional github-linguist dep to a good version](https://github.com/dmarcotte/github-markdown-preview/commit/7b7c0fb10bc1ca4771205d00d9564bf832e7fda9) (the latest versions are currently very cranky...)