Add comma to not condition to match json format.

Author: The-inside-man

core-api/src/main/java/com/optimizely/ab/config/audience/NotCondition.java

@@ -57,7 +57,7 @@ public String getOperandOrId() {
     @Override
     public String toJson() {
         StringBuilder s = new StringBuilder();
-        s.append("[\"not\"");
+        s.append("[\"not\", ");
         s.append(condition.toJson());
         s.append("]");
         return s.toString();