v0.9.15

[daveshanley]

Bugfix release

Addresses the following issues:

• Data race in rule_applicator.go #487
• Panic when I use an external reference as a request body. #486
• Invalid Error when parsing a schema with a field type string + format #482
• Valid spec is mentionned as invalid #481

Also addresses issue with the -m flag on the lint command failing.

Updated libopenapi and libopenapi-validator and doctor versions.