GRADLE-3107 - Make it possible to modify the cache period for missing modules and artifacts

This is a fix for GRADLE-3107 (http://issues.gradle.org/browse/GRADLE-3107), making it possible
to configure the time that Gradle caches the fact that a dependency does not exist in a repository.
The reason for this (as explained in the Jira issue), is that if you have a build with multiple
repositories (and a slow connection to the repositories), a build might take a long time. When
the dependencies are all cached, you'd expect the build to be fast, especially if all the
dependencies are released versions. However, with Gradle's default of re-checking every 24 hours,
the build will take a long time once per 24 hours.

With this change, it's possible to change this default, using:

cacheMissingModulesAndArtifactsFor 500, 'days'