Upgrade maven-site-plugin to version 3.3 to fix broken "mvn site" on Maven 3.2.1.

Related issue: https://github.com/maven-nar/nar-maven-plugin/issues/108
Merge pull request #113 from forderud/master

Upgrade maven-site-plugin to version 3.3 to fix broken "mvn site" on Maven 3.2.1