fix(stepfunctions): Actually perform rendering of NotCondition

Without this fix, the `Not` condition would render as (notice that `variable`, `comparisonOperator` and `value` are not capitalized):
```
{ Not:
   VariableComparison { variable: '$.a', comparisonOperator: 0, value: 'b' } }
```

packages/@aws-cdk/aws-stepfunctions/lib/condition.ts

@@ -221,7 +221,7 @@ class NotCondition extends Condition {
 
  public renderCondition(): any {
  return {
- Not: this.comparisonOperation
+ Not: this.comparisonOperation.renderCondition()
  };
  }
-}
+}

packages/@aws-cdk/aws-stepfunctions/test/test.condition.ts

@@ -8,5 +8,17 @@ export = {
  });
 
  test.done();
- }
+ },
+ 'NotConditon must render properly'(test: Test) {
+ // GIVEN
+ const condition = stepfunctions.Condition.not(stepfunctions.Condition.stringEquals('$.a', 'b'));
+
+ // WHEN
+ const render = condition.renderCondition();
+
+ // THEN
+ test.deepEqual(render, {Not: {Variable: '$.a', StringEquals: 'b'}});
+
+ test.done();
+ },
 };