Use git version of rugged

You need to make sure the correct extension binary gets used. If you
use bundler <= 1.6.1 (where the fix for #2947 was released) and used
latest version of rugged from gem, switching to git rugged will build
the new one but not copy it to the final location and you'll end up
using the old extension (resulting in a superclass mismatch error).

Either upgrade bundler or delete the built extension from
GEM_BASE/bundler/gems/extensions.