Maintenance release

Bugfixes:

• fixing the behavior or "defeault" keyword in "oneOf" schemas - contributed by @dab-libs
• fixing error message for "type": "integer" violations - contributed by @zgyorffi
• fixing NPE while validating "additionalItem" when "items" is not specified - contributed by @zgyorffi
• keep precision in case of big numbers - contributed by @zgyorffi

Enhancements:

• migrating to JUnit 5 - contributed by @gstoupis