Update prerequisites to Maven 3.0.1 (required by findbugs-maven-plugin)
Update Maven plugins

* graylog2-parent
maven-assembly-plugin .......................... 2.4 -> 2.4.1
maven-jar-plugin ............................... 2.4 -> 2.5
maven-javadoc-plugin ........................... 2.9 -> 2.9.1
maven-release-plugin ........................... 2.4.1 -> 2.5
maven-shade-plugin ............................. 2.1 -> 2.3
maven-site-plugin .............................. 3.2 -> 3.4
maven-source-plugin ............................ 2.2.1 -> 2.3
maven-surefire-plugin .......................... 2.13 -> 2.17

* graylog2-rest-routes
org.codehaus.mojo:build-helper-maven-plugin .... 1.8 -> 1.9
org.codehaus.mojo:exec-maven-plugin ............ 1.2.1 -> 1.3.2