[IconMenu] Removed props.ref call

Calls to props.ref will throw a warning in React v15.  This commit hard-codes the
value instead of reading props.ref.  The iconButtonRef element has been removed
from the state object, since it never changes after the component is initialized.

React warnings discussed in this PR:
facebook/react#5744

src/IconMenu/IconMenu.js

@@ -171,7 +171,6 @@ class IconMenu extends React.Component {
   };
 
   state = {
-    iconButtonRef: this.props.iconButtonElement.props.ref || 'iconButton',
     menuInitiallyKeyboardFocused: false,
     open: false,
   };
@@ -205,7 +204,7 @@ class IconMenu extends React.Component {
     this.setState({open: false}, () => {
       // Set focus on the icon button when the menu close
       if (isKeyboard) {
-        const iconButton = this.refs[this.state.iconButtonRef];
+        const iconButton = this.refs.iconButton;
         ReactDOM.findDOMNode(iconButton).focus();
         iconButton.setKeyboardFocus();
       }
@@ -293,7 +292,7 @@ class IconMenu extends React.Component {
         this.open(Events.isKeyboard(event) ? 'keyboard' : 'iconTap', event);
         if (iconButtonElement.props.onTouchTap) iconButtonElement.props.onTouchTap(event);
       },
-      ref: this.state.iconButtonRef,
+      ref: 'iconButton',
     });
 
     const menu = (