JCBC-612: Temporarly disable checkstyle in build

Motivation
----------
Having checkstyle plugin activated in gradle build but no checkstyle
configuration yet causes the build to fail.

Modifications
-------------
Disable checkstyle plugin in gradle build until a checkstyle config is
chosen and put in place.

Result
------
Build doesn't fail due to checkstyle errors anymore.

Change-Id: Ibe6ce6e2d864b5a33c819252bd7b5957c1a85f94
Reviewed-on: http://review.couchbase.org/42654
Tested-by: Simon Baslé <simon@couchbase.com>
Reviewed-by: Michael Nitschinger <michael.nitschinger@couchbase.com>