Used fixed versions for build init templates

Gradle builds started failing on every branch on Java 7 two days ago when TestNG released a new version (6.9.13) built with Java 8.
java.lang.UnsupportedClassVersionError: org/testng/ITestListener : Unsupported major.minor version 52.0

This can be seen e.g. here on CI: https://builds.gradle.org/viewLog.html?buildId=1322067

The root cause of this is that we use dynamic versions (latest.release) to resolve versions of librairies used in the build init templates. This is dangerous.
See https://github.com/gradle/gradle/blob/master/subprojects/build-init/build-init.gradle#L39-L84

As a quick fix for 3.1 we use fixed versions on the release branch. We need to think more holistically about that for
3.2.

gradle/gradle-private#302
+review REVIEW-6252

Publish version 3.1 RC1

Fix test

Publish version 3.0

Publish version 3.0 RC2

Update gradle wrapper version

Update gradle wrapper version

Publish version 3.0 RC1

Remove test flakiness by allowing initial scala compilation to take l…

…onger

The test waits 20s for any build activity, but initial scala compilation can
take up to 1 minute to complete. This makes this tests fail for reasons
completely unrelated to the feature being tested (scala continuous build).

Bumping the timeout should make this test non-flaky, but it will leave the
underlying problem that Scala compilation sometimes is exceedingly slow.