Use ActiveSupport Concern

It makes easy to use version class with different AR connection
Use ClassMethods module for AS::Concern

including missed class methods
Add rspec for concern
Adjust formatting on comments and on README for aesthetics. [ci skip]
Merge remote-tracking branch 'chulkilee/use-concern'
Reorganizing specs for new VersionConcern module merged from #289
Cleaning up the specs for the RSpec helper
Explicitly require ActiveSupport as a dependency; bump to 3.0.0.rc1 [ci skip]
Increase version number on README for Sinatra directions [ci skip]
Simplifying the :reify method a bit on the VersionConcern by removing one level of nested blocks
Fixing a typo on the version_spec
Explicitly specify table name for scope methods on VersonConcern. close #295
Check instance value of :paper_trail_enabled_for_model in Model::InstanceMethods.paper_trail_switched_on?
Fixing state leak issue for :enabled_for_controller? method with RSpec and Cucumber helpers. close #293
Bump to 3.0.0.rc2 [ci skip]
Fix version number in README for Sinatra instructions [ci skip]
Fix link to 2.7-stable branch in README. close #298 [ci skip]
Simplifying query example on the README [ci skip]
typofix before in cucumber should be Before
PaperTrail::VERSION should be inspectable