Update gradle version from 3.3 to 4.0

With Android Studio 3 the app cannot be built at the moment. This
is caused by Gradle 3.3 not supporting the forTask() method.
Updating gradle to 4.0 resolves the issue.

Author: tobias47n9e

gradle/wrapper/gradle-wrapper.properties

@@ -3,4 +3,4 @@ distributionBase=GRADLE_USER_HOME
 distributionPath=wrapper/dists
 zipStoreBase=GRADLE_USER_HOME
 zipStorePath=wrapper/dists
-distributionUrl=https\://services.gradle.org/distributions/gradle-3.3-all.zip
+distributionUrl=https\://services.gradle.org/distributions/gradle-4.0-milestone-1-all.zip