Require Maven 3.3.1+ and introduce mvnw

There are three closely related things here:

• Maven 3.3.1+ is required by the Enforcer Plugin

• mvnw (a.k.a. Maven Wrapper) is a script that downloads and installs
  (if necessary) the required Maven version to ~/.m2/wrapper and runs it
  from there. "mvn -N io.takari:maven:wrapper" was used to create all the
  necessary files. The same command can be used in the future to upgrade
  to a newer wrapper version. Maven version used by the wrapper is stored
  in .mvn/wrapper/maven-wrapper.properties

• .mvn/jvm.config is a place where Maven 3.3.1+ looks for JVM parameters
  (memory, etc.) which were traditionally defined in the MAVEN_OPTS
  environment variable.