Include path configuration for wkhtmltopdf-binary gem

Just spent a good hour trying to find this solution.  Most of the options I saw gave the instruction "put in the right path".  I figured there was a way to grab the 'right path', but wasn't quite well versed to figure it out.  I almost gave up trying to figure it out and went the route of downloading the binary, then extracting it, and putting it into my /bin.  Not very future proof.

 This will automatically update the path when the gem is updated, and the script it points to works by looking at the system to determine the correct binary.  Much cleaner.

Hope it helps!
RubyGems has a function called `Gem.bin_path` for these kinds of things.
This could be simplified to: `Gem.bin_path('wkhtmltopdf-binary', 'wkhtmltopdf')`