Don't require the wkhtmltopdf gem

It can be added/gem installed as needed. Also sort dependencies :).

One possible issue: will it need to be added to the gemfile/gemspec for
it to be picked up when running rake? Or will you be able to just `gem
install wkhtmltopdf`?
Merge remote-tracking branch 'origin/jh-fix-travis'

Conflicts:
	.travis.yml