Add enforcer plugin to break the build earlier if wrong java version …

[akuhtz]

…is used.

[INFO] ------------------------< com.pi4j:pi4j-parent >------------------------
[INFO] Building Pi4J :: Parent POM 2.0-SNAPSHOT                          [1/10]
[INFO] --------------------------------[ pom ]---------------------------------
[INFO]
[INFO] --- maven-clean-plugin:2.5:clean (default-clean) @ pi4j-parent ---
[INFO]
[INFO] --- maven-enforcer-plugin:3.0.0-M2:enforce (enforce-java) @ pi4j-parent ---
[WARNING] Rule 0: org.apache.maven.plugins.enforcer.RequireJavaVersion failed with message:
Detected JDK Version: 9.0.4 is not in the allowed range 11.