Drop support for Ubuntu 14.04 in version 4

Would allow us to:

* remove Supervisor support
* run servers under Java 8 everywhere

The plan would look something like this:

1. Deprecate Ubuntu 14.04 and Supervisor in the next minor release. Use `debug` to issue a deprecation warning.
2. Drop support for both in version 4.

Spigot support isn't live yet, so we don't have to worry about that for Ubuntu 14.04 users.