Adding support for PostgreSQL's JSON column type
Fixing check for whether the object column is of type json in PaperTrail::Model::InstanceMethods.object_attrs_for_paper_trail
Upcasing the provided YAML and JSON serializers
Cleanup code dealing with handling of columns of type :json on pg
Merge branch 'json_column_support'. close #287

Conflicts:
	CHANGELOG.md
Removing before_install block from .travis.yml since it is no longer necessary