Remove wkhtmltopdf-binary dependancy

I recommend removing the wkhtmltopdf-binary dependency because it creates unnecessary bloat since it contains binaries irrelevant to the current platform.

I updated the docs to include info about adding the wkhtmltopdf-binary gem so users can still get up and running quickly. I kept the wkhtmltopdf-binary gem in the Gemfile for testing purposes.

This change is important in my case because I'm using my own binary of wkhtmltopdf 0.11.0_rc1 since it contains some performance improvements, and it also fixes a bug that allows anchor links to work within pdfs. Using wisepdf with wkhtmltopdf 0.11.0_rc1 I haven't run into any issues and the tests still pass when specifying that binary.