Move Gson to static in Keyboard json serialization

Author: pengrad

src/main/java/com/pengrad/telegrambot/model/request/Keyboard.java

@@ -8,7 +8,7 @@
  */
 public abstract class Keyboard {
 
-    private Gson gson = new Gson();
+    private static Gson gson = new Gson();
 
     @Override
     public final String toString() {