TouchableHighlight needes to set _isMounted false when it unmounts.

Summary:
D5016368 to suppress the warning had a typo which meant `_isMounted` would never get set
`false` and thus some functions could be called on unmounted refs.

Reviewed By: yungsters

Differential Revision: D5034076

fbshipit-source-id: 6334db6ee2f9e19c1bb4da2572987dc10773e28d

Libraries/Components/Touchable/TouchableHighlight.js

@@ -149,7 +149,7 @@ var TouchableHighlight = React.createClass({
     ensureComponentIsNative(this.refs[CHILD_REF]);
   },
 
-  componentWillMount: function() {
+  componentWillUnmount: function() {
     this._isMounted = false;
   },