Manually set wkhtmltopdf binary location
Manually set wkhtmltopdf binary location

The gem couldn't find the binary, because it was forking out a call to
`bundler exec which wkhtmltopdf`.

This is a better method and fixes the error when trying to render PDF's.

ausaccessfed/validator-service,ausaccessfed/validator-service,ausaccessfed/validator-service