Don't use unique indexes with polymorphics

With a mix of scoped and unscoped slugs, because the indexes are created
on the top-level class, you'll end up with `duplicate key error`s on the
unscoped index even when scoped slugs would be valid.
Merge pull request #137 from pdf/non_unique_indexes

Don't use unique indexes with polymorphics
Added credits for bugfix to pdf
Added credits for refactoring of testcases to lucasrenan
bumped version
update dependencies
Fix for when using localized slug with custom slug building strategy on virtual attribute.
Merge pull request #141 from astjohn/fix_i18n

Fix for when using localized slug with custom slug building strategy on ...
Give credit to astjohn for bugfix
Fixed compatibility with Mongoid 4.x.
Merge pull request #146 from dblock/mongoid-4x

Fixed compatibility with Mongoid 4.x.
Give credit to dblock for Mongoid 4 support