Bump version to 3.0.6 for release

[maven-release-plugin] copy for tag jruby-maven-plugins-3.0.5

[maven-release-plugin] copy for tag jruby-maven-plugins-3.0.4

[maven-release-plugin] copy for tag jruby-maven-plugins-3.0.3

[maven-release-plugin] copy for tag jruby-maven-plugins-3.0.2

[maven-release-plugin] copy for tag jruby-maven-plugins-3.0.1

[maven-release-plugin] copy for tag jruby-maven-plugins-3.0.0

[maven-release-plugin] copy for tag jruby-maven-plugins-2.0.1

[maven-release-plugin] copy for tag jruby-maven-plugins-2.0.0-1

[maven-release-plugin] copy for tag jruby-maven-plugins-2.0.0