Fix TypeError in componentWillUnmount

this.__viewSpring does not exists, which was resulting in a TypeError.

Author: Matt Hinz

src/navigation-controller.jsx

@@ -112,6 +112,12 @@ class NavigationController extends React.Component {
     });
   }
 
+  componentWillUnmount() {
+    delete this.__springSystem;
+    this.__spring.removeAllListeners();
+    delete this.__spring;
+  }
+
   componentDidMount() {
     // Cache the view wrappers
     this['__view-wrapper-0'] = React.findDOMNode(this.refs[`view-wrapper-0`]);
@@ -124,10 +130,6 @@ class NavigationController extends React.Component {
     });
   }
 
-  componentWillUnmount() {
-    this.__viewSpring.destroy();
-  }
-
   /**
    * Translate the view wrappers by a specified percentage
    *