Remove DoubleCheckedLocking check from Checkstyle config

The `DoubleCheckedLocking` check is not supported by Checkstyle 5.6 which is the default version used in Gradle 1.4.

Also see the [Checkstyle 5.6 release notes](http://checkstyle.sourceforge.net/releasenotes.html#Release_5.6) and the [Gradle 1.4 release notes](http://www.gradle.org/docs/current/release-notes#updated-default-versions-of-checkstyle-and-codenarc).