* Bump gradle wrapper to version 5.6.4
* Add distributionSha256Sum to gradle-wrapper.properties
Gradle will self-verify the sha of the downloaded wrapper
zip upon its first download.
Commit 4d4787d updated the Gradle wrapper config from 2.0 => 2.1, but
did not actually update the wrapper properties file (forgot to run
`gradle wrapper`). This commit finishes what was started there.