Revert "Simplify loading by using on ActiveSupport lazy-load hooks"

This reverts commit 353ac3bf56748a32d8c6ec59752d627491bfbdcf.
Use `require` instead of `autoload` for version.rb
Lazy-load ActiveRecord when used outside of Rails
Use maruku instead of redcarpet for markdown in documentation (JRuby support)
Remove Rails as development dependency (for now)
Use `gemspec` for DRYing-up CI gemfiles
Add a CI gemfile for ActiveRecord 4 Release Candidate
Refactor spec_helper.rb to share SimpleCov settings between development and CI environments