* [gradle] Add recommendations to validate task
* Use current version of Gradle Plugin in build checks.
* Fix gradle project build version confusion in CI
* [gradle] Bump samples to 5.2.1 wrapper
Previously, the Gradle plugin was building in CI against
openapi-generator 4.2.0 and Gradle version 4.10.2. At some point, a
contribution was made with an API which is incomatible at 4.10.2 and due
to a release script error which pinned the local-spec version to release
4.2.0, we didn't notice this inconsistency.
This bumps the project to build against Gradle 5.2.1.
* Explicitly ignore .bak in gradle samples directory
* Add gradle wrapper to local-spec sample, make .java-version (jenv local) be less machine specific
* Include gradle plugin samples in travis build
