Remove Everit dependency

[travisbrown]

The immediate goal for this library is to allow us to avoid referring to Everit validator types directly in our internal projects at Permutive, and the 0.1 release accomplishes this goal.

There are two next steps: to identify what other functionality is necessary for other potential adopters, and to remove the Everit dependency, which will allow us to avoid the non-Maven Central resolver as well as the runtime overhead of converting to the org.json JSON representation.