fix to avoid VersionError on save model

Using *mongoose-versioner* with *mongoose v3* or later,
`VersionError` occurs when repeatedly save a model.

It is because *mongoose v3* automatically adds `versionKey` (default "__v") to a model,
and this plugin copies versionKey into another model.

To avoid this problem, I fixed codes to not copy `versionKey` of a model.
This fix is mongoose v2 compatible yet.