bump version to 0.4.20

several improvements to gem and gemspec (#98)

* Upgrade jackson and jackson-databind to v2.15.4
* Build using JRuby 9.4.7.0 and Junit 4.13.2
* Update development gems such as json 2.x
* Remove dependency on git ls-files in gemspec
* Remove unnecessary content from gem such as .java files and tests

bump jackson to 2.15.2 (#92)

Version 0.4.17

upgrade jackson to 2.13.3 (#87)

Bump jackson databind to 2.9.10.8 (#84)

Version 0.4.11

Version 0.4.9

Version 0.4.8

Version 0.4.7