1.13.9

A big upgrade of the plugin in order support 1.13.
Please do not use your previous arena configurations and re-create it for a better compatibility.
If you are an experienced user of the plugin and if you are brave, you can upgrade from previous version even so. Please read this documentation : https://github.com/Eredrim/pvparena/blob/master/doc/update-from-1-3-x.md