Support new Multiverse-Core API

[Wyatt-James]

POM

• Multiverse-Core is now fetched from its official maven repository instead of packaged in the bChatManager lib directory. Removed the now-unused JAR.
• Vault is now fetched from its new maven repository, as the old one no longer exists.
• Bumped Vault version to 1.7.1 as the new repository does not appear to host 1.4.1.
• Bumped java source and target from 1.6 to 1.8 to appease the great and mysterious maven.
• Bumped Multiverse-Core version from 2.5 to 5.4.0.

Plugin

• Updated to use the new Multiverse-Core API.

Misc

• Bumped bChatManager version from 3.1 (3.1.6 in plugin.yml) to 4.0.0, as the other changes were breaking.

closes #11, closes #12

[Wyatt-James]

I'd like an opinion on the bChatManager version bump, since I don't know how that was handled prior.