Update Gradle wrapper to use 2.6 [ci skip]

Author: lukespragg

gradle/wrapper/gradle-wrapper.properties

@@ -2,4 +2,4 @@ distributionBase=GRADLE_USER_HOME
 distributionPath=wrapper/dists
 zipStoreBase=GRADLE_USER_HOME
 zipStorePath=wrapper/dists
-distributionUrl=http\://services.gradle.org/distributions/gradle-2.0-all.zip
+distributionUrl=http\://services.gradle.org/distributions/gradle-2.6-all.zip