Move maven wrapper to the proper directory in the core project

.gitignore

@@ -56,4 +56,4 @@
 /.project
 .idea
 *.iml
-.mvn/wrapper/maven-wrapper.jar
+/core/.mvn/wrapper/maven-wrapper.jar

.mvn/wrapper/MavenWrapperDownloader.java → core/.mvn/wrapper/MavenWrapperDownloader.java

@@ -1,3 +1,18 @@
+/**
+ *    Copyright 2006-2018 the original author or authors.
+ *
+ *    Licensed under the Apache License, Version 2.0 (the "License");
+ *    you may not use this file except in compliance with the License.
+ *    You may obtain a copy of the License at
+ *
+ *       http://www.apache.org/licenses/LICENSE-2.0
+ *
+ *    Unless required by applicable law or agreed to in writing, software
+ *    distributed under the License is distributed on an "AS IS" BASIS,
+ *    WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ *    See the License for the specific language governing permissions and
+ *    limitations under the License.
+ */
 /*
 Licensed to the Apache Software Foundation (ASF) under one
 or more contributor license agreements.  See the NOTICE file