fix: Trigger custom click handler in collapsible nodes

There was a mistake in the `_tick` method which caused the user's
`onNodeClick` custom handler to never be triggered when the collapsible
feature was enabled.

The problem was that it called `setState` without using a callback
parameter if the received `cb` was not empty. The contents of that
ternary operator should be the opposite for things to work.

This fix closes #136.

src/components/graph/Graph.jsx

@@ -185,7 +185,7 @@ export default class Graph extends React.Component {
      * @param {Function} [cb] - optional callback to fed in to {@link setState()|https://reactjs.org/docs/react-component.html#setstate}.
      * @returns {undefined}
      */
-    _tick = (state = {}, cb) => (cb ? this.setState(state) : this.setState(state, cb));
+    _tick = (state = {}, cb) => (cb ? this.setState(state, cb) : this.setState(state));
 
     /**
      * Configures zoom upon graph with default or user provided values.<br/>