update way to set language level.

Author: hideki

build.gradle

@@ -2,8 +2,8 @@ apply plugin: 'java'
 apply plugin: 'maven'
 apply plugin: 'eclipse'
 
-sourceCompatibility = 1.6
-targetCompatibility = 1.6
+sourceCompatibility = JavaVersion.VERSION_1_6
+targetCompatibility = JavaVersion.VERSION_1_6
 
 repositories {
     maven { url 'http://files.couchbase.com/maven2/' }