Make the versionsTable use types.JSONText again

[arschles]

its Data field had to be a string in #123 because of the panic described in #123 (comment). Since clustersCheckinsTable uses a types.JSONText, it should be possible to use one in this field as well

[arschles]

I'm moving this to RC1

[mboersma]

@arschles is this still a change we need to make in WFM?