Versions rollback

This PR updates version rollback function to exclude ignorePaths fields in the rollback operation


Please also review ignorePaths incorporated in versioned schema so that these fields are excluded on the schema level.
https://github.com/flashmedia/mongoose-version/commit/0deb2956dfaadaca4a6d548b9c9bb33844430f34 

@smebberson This is ready for review