Form version should only be numbers

[batkinson]

Per the documentation on xlsform.org, the version setting form a form is:

version: String of up to 10 numbers that describes this revision. Revised form definitions must have alphabetically greater versions than previous ones. A common convention is to use strings of the form ‘yyyymmddrr’. For example, 2017021501 is the 1st revision from Feb 15th, 2017.

Currently, the api allows any string. The api should provide some means of intercepting or failing on these types of issues at a minimum.