Add warning for Java 7

Author: stephan-gh

Core/src/main/java/net/minecrell/serverlistplus/core/ServerListPlusCore.java

@@ -82,6 +82,18 @@ public ServerListPlusCore(ServerListPlusPlugin plugin) throws ServerListPlusExce
 
         getLogger().log(INFO, "Starting...");
 
+        try {
+            if (Float.parseFloat(JAVA_CLASS_VERSION.value()) < 52.0) {
+                getLogger().log(WARN, "You're using Java 7 or lower. Keep in mind future versions of ServerListPlus " +
+                        "may require Java 8 or higher. Java 8 contains various improvements, especially regarding " +
+                        "performance. If possible, please upgrade as soon as possible.");
+            }
+        } catch (NumberFormatException ignored) {
+            getLogger().log(WARN, "Failed to detect Java version. If you're using Java 7 or lower, keep in mind " +
+                    "future versions of ServerListPlus may require Java 8 or higher. Java 8 contains various " +
+                    "improvements, especially regarding performance. If possible, please upgrade as soon as possible.");
+        }
+
         // Print some information about the environment
         getLogger().log(REPORT, Helper.joinLines(
                 "Plugin Information:",