Add server version compatibility check.

Instead of the obscure error thrown from ArenaClass when the SHIELD enum value on Material isn't found, MobArena throws its own, more human-friendly error.

Fixes #469

src/main/java/com/garbagemule/MobArena/MobArena.java

@@ -69,6 +69,8 @@ public class MobArena extends JavaPlugin
     }
 
     public void onEnable() {
+        ServerVersionCheck.check(getServer());
+
         // Initialize config-file
         configFile = new File(getDataFolder(), "config.yml");
         config = new YamlConfiguration();

src/main/java/com/garbagemule/MobArena/ServerVersionCheck.java

@@ -0,0 +1,44 @@
+package com.garbagemule.MobArena;
+
+import org.bukkit.Server;
+
+import java.util.Arrays;
+import java.util.StringJoiner;
+
+class ServerVersionCheck {
+
+    private static final String[] EXACTS = {"1.11", "1.12"};
+    private static final String[] PREFIXES = {"1.11.", "1.12."};
+
+    static void check(Server server) {
+        String version = getMinecraftVersion(server);
+
+        for (String exact : EXACTS) {
+            if (version.equals(exact)) {
+                return;
+            }
+        }
+        for (String prefix : PREFIXES) {
+            if (version.startsWith(prefix)) {
+                return;
+            }
+        }
+
+        throw new IllegalStateException(new StringJoiner(" ")
+            .add("Incompatible server version!")
+            .add("This build only works on " + Arrays.toString(EXACTS) + ",")
+            .add("but this server is running " + version + ".")
+            .add("Perhaps you downloaded the wrong build?")
+            .toString()
+        );
+    }
+
+    private static String getMinecraftVersion(Server server) {
+        // Same substring as the one bStats uses, so should be safe
+        String version = server.getVersion();
+        int start = version.indexOf("MC: ") + 4;
+        int end = version.length() - 1;
+        return version.substring(start, end);
+    }
+
+}