Revert "Remove duplicate dependency"

This reverts commit 09aeccfbc8aca78771222a66c4f207f19a1ce50f.
Remove duplicate dependency the other way around
Update Gemfile.lock
Version bump to 0.1.27
Regenerate gemspec for version 0.1.27
Update changelog for 0.1.27
Merge branch 'master' into polymorphic-assosiations

Conflicts:
	Gemfile
Added some spec examples for polymorpic two-level assosiations
Fixed update polymorphic test