Reference version from force.go

I'm not sure if there was a reason for not doing this, but I figured I'd put it out there in case you wanted to merge.

Instead of having "28.0", "29.0", "30.0" strewn around, we can
keep the version in a single file and then reference that from
elsewhere to make upgrading and changing Api versions easy

Excluded push.go because there is another patch covering that one