Should've been null checked since I made it nullable. String concat
saved us any real damage and so all we saw was "null" shown to users.
Quick check and now we don't.
This makes it easier for downstream projects (forks) to replace the
version fetching system with their own. It is as simple as implementing
an interface and overriding the default implementation of
org.bukkit.UnsafeValues#getVersionFetcher()
It also makes it easier for us to organize things like the version
history feature.
Lastly I have updated the paper implementation to check against the site
API rather than against jenkins.
