Switch to Lo-Dash from Underscore

Fixes the following issue:
https://github.com/TypeStrong/grunt-ts/issues/117

Underscore is still required due to underscore.string usage in a few places.

I ran the tests and they pass.

The original issue gives reasoning for the change. Lo-Dash is preferred over Underscore by most people these days, and this should increase grunt-ts performance a bit.