Use a blacklist for the server version check.

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.

src/test/java/com/garbagemule/MobArena/ServerVersionCheckTest.java

@@ -0,0 +1,74 @@
+package com.garbagemule.MobArena;
+
+import org.bukkit.Server;
+import org.junit.Before;
+import org.junit.Rule;
+import org.junit.Test;
+import org.junit.rules.ExpectedException;
+import org.junit.runner.RunWith;
+import org.mockito.junit.MockitoJUnitRunner;
+
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.when;
+
+@SuppressWarnings("WeakerAccess")
+@RunWith(MockitoJUnitRunner.StrictStubs.class)
+public class ServerVersionCheckTest {
+
+    public Server server;
+
+    @Rule
+    public ExpectedException exception = ExpectedException.none();
+
+    @Before
+    public void setup() {
+        server = mock(Server.class);
+    }
+
+    @Test
+    public void oneFourteen() {
+        win("git-Spigot-cafebae-dedbeef (MC: 1.14)");
+    }
+
+    @Test
+    public void oneThirteen() {
+        win("git-Spigot-f09662d-be557e6 (MC: 1.13.2)");
+    }
+
+    private void win(String version) {
+        when(server.getVersion()).thenReturn(version);
+        ServerVersionCheck.check(server);
+    }
+
+    @Test
+    public void oneTwelve() {
+        fail("git-Spigot-79a30d7-acbc348 (MC: 1.12.2)");
+    }
+
+    @Test
+    public void oneEleven() {
+        fail("git-Spigot-3fb9445-6e3cec8 (MC: 1.11.2)");
+    }
+
+    @Test
+    public void oneTen() {
+        fail("git-Spigot-de459a2-51263e9 (MC: 1.10.2)");
+    }
+
+    @Test
+    public void oneNine() {
+        fail("git-Spigot-c6871e2-0cd0397 (MC: 1.9.4)");
+    }
+
+    @Test
+    public void oneEight() {
+        fail("git-Spigot-21fe707-e1ebe52 (MC: 1.8.8)");
+    }
+
+    private void fail(String version) {
+        when(server.getVersion()).thenReturn(version);
+        exception.expect(IllegalStateException.class);
+        ServerVersionCheck.check(server);
+    }
+
+}