No longer test on JDK 6

… it's time to move forward.
Update license date
Update maven-site-plugin to 3.3
Updated maven-project-info-reports-plugin to 2.7
Use correct SCM identifier in POM
Fix JavaDoc link in MavanagaiataMojoException.create
Link to the JavaDocs of the correct JGit version
Link to the JavaDocs of Java 7
Improve and test JGitRepository#check()
Version bump to 0.6.0