.gitignore: !gradle/wrapper/gradle-wrapper.jar
add gradle/wrapper/gradle-wrapper.jar

Without this, Heroku will failed to deploy:

-----> Gradle app detected
-----> Installing OpenJDK 1.6... done
-----> Building Gradle app...
       WARNING: The Gradle buildpack is currently in Beta.
-----> executing ./gradlew stage
       Exception in thread "main" java.lang.NoClassDefFoundError: org/gradle/wrapper/GradleWrapperMain
       Caused by: java.lang.ClassNotFoundException: org.gradle.wrapper.GradleWrapperMain
        at java.net.URLClassLoader$1.run(URLClassLoader.java:217)
        at java.security.AccessController.doPrivileged(Native Method)
        at java.net.URLClassLoader.findClass(URLClassLoader.java:205)
        at java.lang.ClassLoader.loadClass(ClassLoader.java:321)
        at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:294)
        at java.lang.ClassLoader.loadClass(ClassLoader.java:266)
       Could not find the main class: org.gradle.wrapper.GradleWrapperMain. Program will exit.
 !     Failed to build app

 !     Push rejected, failed to compile Gradle app
build.gradle: add task stage

Heroku needs it