Document minimum memory requirement for Gradle builds

The build failed on two classes from spring-xd-extension-process:
ShellCommandProcessor.java and ShellCommandProcessorTests.java
with error:
FAILURE: Build failed with an exception.

• What went wrong:
  Execution failed for task ':spring-xd-extension-process:licenseTest'.
  > License violations were found: /Users/victor.chugunov/git/repos/spring/spring-xd/extensions/spring-xd-extension-process/src/test/java/org/springframework/xd/extension/process/ShellCommandProcessorTests.java}

In both classes the license is misplaced:
package org.springframework.xd.extension.process;/*
*

• * Copyright 2014 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.
  *
  */