Release v0.3.1-stable

Works with 1.7.0 and 1.7.1

What's Changed

• Added checks on server cleanup to prevent shutdown NPE
• Fixed gym leave NPE
• Fixed datagen saving recipes in wrong namespace

Full Changelog: https://github.com/gitoido-mc/rad-gyms/compare/1.7.0+0.3.0-stable...1.7.0+0.3.1-stable