Update to use gradle v2.1

* Updated gradle-wrapper.properties to use gradle v2.1
* Updated build.gradle to use buildscript v0.13.2
* Fix NPE at pom.version
* Update couchbase-lite-java-core submodule to link to the corresponding fix commit

#461
Link to gradle binary only archive
Merge pull request #462 from couchbase/issue_461_gradle_2_1

Update to use gradle v2.1