Bumping maven-site-plugin to 3.3 to work with Maven 3.1.1

Bumping maven-site-plugin to 3.3 to work with Maven 3.1.1

* https://bugzilla.mozilla.org/show_bug.cgi?id=949716
* https://github.com/jacoco/jacoco/issues/121
* http://search.maven.org/#artifactdetails|org.apache.maven.plugins|maven-site-plugin|3.3|maven-plugin