Added support for Rails 4

Changes:
* use built-in Rails database adapter instead of old serialize call
* update to use new `scope` syntax that requires a lambda
* simplify code by using ActiveSupport::Concern
* DRY up declaration of "true" value
* fix a bug that requires `("#{field}_will_change!"` to be called
  before mutating the actual flags hash
* remove unnecessary bundler Gemfile entries