RFR: Add gradle wrapper properties file to help IDEs

[Robin Westberg]

Hi all,

Please review this change that adds a gradle-wrapper.properties file that describes the required version of Gradle.
This will make IDEs such as IntelliJ able to open the project out of the box.
Best regards,
Robin

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Commit messages:
 - Add gradle wrapper properties file to help IDEs

Changes: https://git.openjdk.java.net/skara/pull/487/files
 Webrev: https://webrevs.openjdk.java.net/skara/487/webrev.00
  Stats: 23 lines in 1 file changed: 23 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/skara/pull/487.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/487/head:pull/487

PR: https://git.openjdk.java.net/skara/pull/487