Allow globally installed rethinkdb driver

[IlyaSemenov]

I use rethink-migrate as an external tool to run migrations in a Python project (I installed it with sudo npm install -g rethink-migrate). However, for some reason rethink-migrate explicitly requires a rethinkdb driver to be installed locally by hard-coding this:

var modules = process.cwd() + '/node_modules/';
...
r = require(modules + 'rethinkdb');

Please allow rethink-migrate to pull a globally installed rethinkdb driver as well. For now I have to pollute my project with node_modules just to please rethink-migrate.