Pry::MethodInfo.gem_root: return gem root instead of ruby root

This seems to fix #16.

Still not ideal, though:

* Scanning `active_support` for methods on Kernel seems bad.

* If we don't find anything, we'll try again and again the next time. Luckily, `Pry::MethodInfo.c_files_found?(path/to/active_support)` is fast here. And returns false.

* This still doesn't work with the [Debian packaging structure](https://packages.debian.org/jessie/all/ruby-activesupport-3.2/filelist) (I just looked it up). But whatever, they brought it on themselves. (And `gem_root` won't be called in those cases anyway).