Edit the use ofextend() to work with xtend's latest version

xtend version 1.0.3 used to edit the object in place (make the first arg contain all of the properties), but with version 2.x, xtend creates and returns a new object that has all of the properties. So instead of just calling `extend()`, its return value needs to be reassigned to the first argument (`parameters` and `options`) in `digitialocean.js`.