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();
+    },
 };