Require minimum terraform version

[jaydgruber]

If used with terraform < 0.13 (I used 0.12.31), this will fail with an error like:

Error: Custom variable validation is experimental

  on .terraform/modules/deepmerge/variables.tf line 4, in variable "maps":
    4:     validation {

This feature is currently an opt-in experiment, subject to change in future
releases based on feedback.

Activate the feature for this module by adding variable_validation to the list
of active experiments.

Seems like this should specify a minimum version of 0.13

[KyleKotowick]

Fixed in v0.1.6.