Update praw library

Hi,

I was looking for a quick script to do exactly what your program does, but was faced with a dependency error for update_checker, so the first thing I did was to update praw to see if that solved it, *then* I realized it was update_checker all along.

Since it seems the purpose of the script is to be standalone I've included my changes.

* Update praw to version 2.1.18
* Add update_checker library dependency

Hope that helps, thanks!!