Make it work for new release 0.12.5

[verheyenkoen]

A couple of days ago wkhtmltopdf 0.12.5 has been released. It turns out the buildpack doesn't work out-of-the-box when setting the version in config (the release url seems to have changed). Can you make the buildpack work for the new version?

[turicas]

I also maintain a wkhtmltopdf buildpack - this is a related issue: wkhtmltopdf/wkhtmltopdf#3947 (comment)

[sorentwo]

This should be fixed with #36, though it doesn't appear to be in a mergeable state currently. Closing in favor of the PR.