The "better handle missing or old dependencies" release.

This adds protection against missing gojsonnet and if the jsonschema module is too old to support the "format checker".

If gojsonnet is missing we fall back to the C library which is dramatically slower.

If the old jsonschema is found then any string format constraints will be ignored.

What's Changed

• Update for jsonschema v3.2.0 by @plasorak in #46

Full Changelog: 0.6.6...0.6.7