Missing api-version in add-on modules

[TheLuca98]

The main helper.jar correctly specifies api-version: 1.13 in the plugin.yml, but all the other modules are missing that property, forcing the server to load them in legacy mode unnecessarily.

[lucko]

Should be an easy PR? :)

[TheLuca98]

I was hoping you wouldn't say that :p

#125

[lucko]

Thank youuuu!