Go back to using our own RC4 implementation

By doing so, we are able to have no external development
dependencies for Prawn. This is a low-risk dependency,
but I am a completionist at :heart:.

This reverts commit ceec2ccaf83c0b510dece01dd5a06bd7d1413948.

Resolves Conflicts:

	Gemfile
	lib/prawn/security.rb
	prawn.gemspec