Fixed warning errors on build

This PR fixes the following errors on build:

    [WARNING] 
    [WARNING] Some problems were encountered while building the effective model for com.github.championsdev:champions-bukkit:jar:0.0.1-SNAPSHOT
    [WARNING] 'build.plugins.plugin.version' for org.apache.maven.plugins:maven-jar-plugin is missing. @ line 78, column 17
    [WARNING] 
    [WARNING] It is highly recommended to fix these problems because they threaten the stability of your build.
    [WARNING] 
    [WARNING] For this reason, future Maven versions might no longer support building such malformed projects.
    [WARNING] 
And:

    [WARNING] Using platform encoding (UTF-8 actually) to copy filtered resources, i.e. build is platform dependent!
And finally:

    [WARNING] File encoding has not been set, using platform encoding UTF-8, i.e. build is platform dependent!

These are fixed in library, core-bukkit and core-canary.