update required kapitan version to released version 0.31.0

[vlcinsky]

https://github.com/kapicorp/kapitan-reference/blob/master/.kapitan#L1

Existing .kapitan requires version 0.31.0rc0

As kapitan version 0.31.0 is already released, it seems reasonable to update this requirement to 0.31.0