Use https for repo.gradle.org

[mkmaier]

This fixes downloading of gradle resources, since gradle.org does not allow http anymore

[uschindler]

Hi,
the reason for using HTTP was mainly that forbiddenapis is still built with Java 6. If the Gradle server is one of those that no longer allows TLS 1.0/1.1 we have the problem that it can't be built anyways.
Nevertheless I am looking into that. It's too bad that Gradle is not on Maven Central.

[uschindler]

That's also the reason why there's "ivy.maven-central.backup", which is set by ANT based on Java version.

[mkmaier]

Maybe this would be a good time to drop support for java 6. The current version will still be available, so if there are really users still on java 6, they can just use that.

Note: This PR is only a preparation for what I really want to do, which is adding configuration for the gradle build cache to work properly. This would also mean requiring at least gradle 3.2, which introduces @Classpath.