Upgrade to Gradle 1.6 and resolve deprecations

Before this change, Spring shared Gradle plugins were using Gradle 1.1.
Recently Spring core framework build started using Gradle 1.6. New
Gradle has deprecated several APIs which were used in Spring shared
Gradle plugins, resulting in deprecation warnings being printed in
Spring framework build output itself.

This patch upgrade Gradle to 1.6, and replaces usage of deprecated
Gradle APIs in Spring shared Gradle plugins.

This patch also includes upgrade of Gradle Artifactory Plugin (GAP) to
latest released version, 2.1.0. Unfortunatelly that plugin also uses
deprecated Gradle APIs, but to a lesser extent. When GAP-144 is
resolved, dependency on GAP should be further upgraded.

When merging this patch, please make sure that GAP 2.1.0 release is
available on springsource plugins-release repository.

Issue: GRADLE-53