add checksum for gradle release

gradle/wrapper/gradle-wrapper.properties

@@ -4,3 +4,4 @@ distributionPath=wrapper/dists
 zipStoreBase=GRADLE_USER_HOME
 zipStorePath=wrapper/dists
 distributionUrl=https\://services.gradle.org/distributions/gradle-4.4-all.zip
+distributionSha256Sum=7a2c66d1a78f811d5f37d14630ad21cec5e77a2a4dc61e787e2257a6341016ce