Relax version constraints on plek.
Relax version constraints on plek.

From a review, the changes in plek 0.2 and 0.3 will not cause any issues
here.  Therefore we can allow any of these versions.

theodi/govuk_content_models,dinuksha/Test,theodi/govuk_content_models,dinuksha/Test,alphagov/govuk_content_models