Standardize version increments (and increment version to 2.1)

[dblock]

I propose to standardize version increments to follow semver and add something about this in a new RELEASING.md:

• patch, e.g. 2.0.2 -> 2.0.3, fixes
• minor, e.g. 2.0.2 -> 2.1.0, new features
• major, e.g. 2.0.2 -> 3.0.0, breaking changes

I think re-adding JDK8 and many infrastructure additions qualify as "new features", we should increment the version to 2.1.0.