Fixes #6 - use correct relation symbols

/me ashamed

Author: mateusz-bajorek

src/main/java/com/currencyfair/onesignal/model/notification/Relation.java

@@ -24,9 +24,9 @@
 public enum Relation {
 
     //CHECKSTYLE:Off Javadoc
-    LESS_THAN(">"),
-    GREATER_THAN("<"),
-    EQUALS("+"),
+    LESS_THAN("<"),
+    GREATER_THAN(">"),
+    EQUALS("="),
     NOT_EQUALS("!="),
     EXISTS("exists"),
     NOT_EXISTS("not_exists");