Try requiring build/Debug/nodegit if build/Release/nodegit wasn't found.

I just spent half an hour trying to figure out why nodegit couldn't require `build/Release/nodegit.node`. Turns out I was running version of node that was built with `./configure --debug`, which for some reason makes node-gyp put the compiled binary in another directory, `build/Debug/`.

This fix falls back to looking in the `Debug` directory if `nodegit.node` couldn't be found in the `Release` directory.