RFR: Update to Gradle 6.4.1

Tue Jun 2 09:55:27 UTC 2020

On Tue, 2 Jun 2020 09:47:35 GMT, Erik Helin <ehelin at openjdk.org> wrote:

> Hi all.
> 
> please review this patch that updates Gradle to 6.4.1. I also went over the README to document the versions of tools
> used for building and testing.
> Testing:
> - [x] `make test` passes on Linux x64
> 
> Thanks,
> Erik

Looks good!

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Marked as reviewed by rwestberg (Reviewer).

PR: https://git.openjdk.java.net/skara/pull/635