Use load hook to extend ActiveRecord

[thlacroix]

PR Checklist

PR Structure

• This PR has reasonably narrow scope (if not, break it down into smaller
  PRs).
• This PR avoids mixing refactoring changes with feature changes (split into
  two PRs otherwise).
• This PR's title starts is concise and descriptive.

Thoroughness

• This PR adds tests for the most critical parts of the new functionality or
  fixes.
• I've updated any docs, .md files, etc… affected by this change.

What

Wrapping the ActiveRecord::Base extend call with a ActiveSupport.on_load hook.

Why

https://github.com/fnando/ar-sequence/blob/v0.2.1/lib/ar/sequence.rb#L25 causes ActiveRecord::Base to be loaded too early when the gem is loaded, which breaks some rails configurations on initialization. Cf rails/rails#50133