Gradle plugin does not write all required manifest entries

[philwebb]

The Java buildpack relies on a few manifest entries to detect Spring Boot archives. With the Maven Plugin these are written here.

Since Spring Boot 2.0 the Gradle plugin no longer uses the Repackager class and we've unfortunately lost these entries. We'll likely need to change these lines.

[rhamedy]

@philwebb how is this for a first-timer contribution?

[clevertension]

@philwebb @rhamedy are you working to fix this issue? if not, i can provide a PR 😛