The whitelisting approach works well for the legacy builds, because the
version landscape doesn't change with those. However, the main build on
the master branch will not run on a server version other than 1.13, so
come 1.14, MobArena will be broken.
This commit fixes this problem by changing to a blacklisting approach
for the main build on the master branch. Checking for versions that the
build *can't* run on brings back MobArena's old resilience to version
changes on the Minecraft side of things.
Andreas Troelsen
4f548e08a6