RequireJavaVersion: Support new Java 9 versioning schema even for older Java versions

Although with MENFORCER-274 it is now possible to specify

                <requireJavaVersion>
                  <version>9</version>
                </requireJavaVersion>

it is still not possible to refer to older java versions without the leadering 1..
So e.g. this one does not work

                <requireJavaVersion>
                  <version>6</version>
                </requireJavaVersion>

This would be very handy as with JDK 9 you specify e.g. release only without the preceeding 1. (compare with https://docs.oracle.com/javase/9/tools/javac.htm#GUID-AEEC9F07-CB49-4E96-8BC7-BCC2C7F725C9__GUID-D343F6B4-3FDD-43A8-AD24-43DD70214471). As it is very common to reuse the same Maven property value for enforcing the minimum java version with the maven-enforcer-plugin as well as for specifying the release/source/target of the maven-compiler-plugin it would be nice, if both support the same version range.

To still be backwards compatible I would suggest that in case of 6,7,8 given as a value, the 1. is implicitly prepended by the maven-enforcer-plugin.