I've created a draft PR , mostly from code in the draft Java 11 PR 
to drive the upgrade of NiFi's Groovy dependencies to version 2.5.4. If
you're interested in NiFi+Groovy, please take a look at it. Several POMs
have been updated to clean up some configurations of maven-compiler-plugin,
a migration from groovy-all to individual modules, etc. So far, the build
and tests are passing. More live testing is required, but on a first pass,
it's looking promising.
There's a regression of an issue fixed in Groovy 2.5.4 that has resurfaced
in Groovy 2.5.7, so for now the PR uses Groovy 2.5.4.
Mike Thomsen, we can coordinate to bring in necessary changes from your PR
 and general review, and commence testing.