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 http://checkstyle.sourceforge.net/releasenotes.html#Release_5.6 for the
release notes of Checkstyle 5.6.
Merge pull request #6 from joschi/checkstyle-config

Remove DoubleCheckedLocking check from Checkstyle config