Update Maven plugins to recent versions

maven-clean-plugin 2.4.1 -> 2.5
maven-enforcer-plugin 1.0.1 -> 1.1
maven-plugin-plugin 2.9 -> 3.1
maven-release-plugin 2.2.2 -> 2.3.2
maven-shade-plugin 1.4 -> 1.7.1
maven-site-plugin 2.1.1 -> 3.1
org.codehaus.mojo:animal-sniffer-maven-plugin 1.7 -> 1.8
org.codehaus.mojo:buildnumber-maven-plugin 1.0 -> 1.1

I checked changelogs between versions and upgrade should go smoothly.