Add update warning to /sf versions

pom.xml

@@ -349,7 +349,7 @@
         <dependency>
             <groupId>com.github.baked-libs.dough</groupId>
             <artifactId>dough-api</artifactId>
-            <version>fcdbd45aa0</version>
+            <version>1108163a49</version>
             <scope>compile</scope>
         </dependency>
         <dependency>

src/main/java/io/github/thebusybiscuit/slimefun4/core/commands/subcommands/VersionsCommand.java

@@ -65,11 +65,25 @@ public void onExecute(@Nonnull CommandSender sender, @Nonnull String[] args) {
                 .append(serverSoftware)
                 .color(ChatColor.GREEN)
                 .append(" " + Bukkit.getVersion() + '\n')
-                .color(ChatColor.DARK_GREEN)
+                .color(ChatColor.DARK_GREEN);
+
+            builder
                 .append("Slimefun ")
                 .color(ChatColor.GREEN)
-                .append(Slimefun.getVersion() + '\n')
+                .append(Slimefun.getVersion())
                 .color(ChatColor.DARK_GREEN);
+            if (!Slimefun.getUpdater().isLatestVersion()) {
+                builder
+                    .append(" (").color(ChatColor.GRAY)
+                    .append("Update available").color(ChatColor.RED).event(new HoverEvent(HoverEvent.Action.SHOW_TEXT, new Text(
+                        "Your Slimefun version is out of date!\n" +
+                        "Please update to get the latest bug fixes and performance improvements.\n" +
+                        "Please do not report any bugs without updating first."
+                    )))
+                    .append(")").color(ChatColor.GRAY);
+            }
+
+            builder.append("\n");
             // @formatter:on
 
             if (Slimefun.getMetricsService().getVersion() != null) {

src/main/java/io/github/thebusybiscuit/slimefun4/core/services/UpdaterService.java

@@ -1,6 +1,7 @@
 package io.github.thebusybiscuit.slimefun4.core.services;
 
 import java.io.File;
+import java.util.concurrent.ExecutionException;
 import java.util.logging.Level;
 
 import javax.annotation.Nonnull;
@@ -110,6 +111,29 @@ public int getBuildNumber() {
         return -1;
     }
 
+    public int getLatestVersion() {
+        if (updater != null && updater.getLatestVersion().isDone()) {
+            PrefixedVersion version;
+            try {
+                version = updater.getLatestVersion().get();
+                return version.getVersionNumber();
+            } catch (InterruptedException | ExecutionException e) {
+                return -1;
+            }
+        }
+
+        return -1;
+    }
+
+    public boolean isLatestVersion() {
+        if (getBuildNumber() == -1 || getLatestVersion() == -1) {
+            // We don't know if we're latest so just report we are
+            return true;
+        }
+
+        return getBuildNumber() == getLatestVersion();
+    }
+
     /**
      * This will start the {@link UpdaterService} and check for updates.
      * If it can find an update it will automatically be installed.