close #391;  Model::InstanceMethods#changes_for_paper_trail should return a plain Hash instead of an ActiveSupport::HashWithIndifferentAccess

This will cut down on the length of the Strings being stored in the 'object_changes'
column on the 'versions' table in the database.
Invoke HashWithIndifferentAccess#to_hash to cast to a hash since #to_h is not defined on Ruby19 and lower
Update README.md

Removed spurious apostrophe
Merge pull request #435 from tom-lord/patch-1

Update README.md [ci skip]