Rename model/field property "version" to "minversion"

This is dependent on #263. (versioned serializers)

We use the metadata property "version" to specify the minimum version of a model or model's field.  This property should be called "minversion".  

**Change:**
I made the change on the models with his property, as well as in the versioned serializers code.

**Testing:**
* I ran `generaterealdata`, then `syncmodels`, with this installed on a central and distributed servers.  Syncing ran fine--some indication of no misspellings.