Enforce check against Java 6 API signature

[belingueres]

Add enforcer rule to check for correct usage of Java 6 API. This is useful since:

• Current TravisCI default maven version 3.5.2 requires Java 7 to run.
• This project still is not built with Java 9+ to compile with the maven.compiler.release=6 property.

[rfscholte]

I've pushed it to Java 7