Fix: Tweak fragment tag name

1. Get rid of a warning in Firefox.
2. Use correct tag name for React 15.

Author: sapegin

src/rsg-components/Preview/Preview.js

@@ -9,7 +9,8 @@ import splitExampleCode from '../../utils/splitExampleCode';
 /* eslint-disable no-invalid-this, react/no-multi-comp */
 
 const compileCode = (code, config) => transform(code, config).code;
-const wrapCodeInFragment = code => `const __f = React.Fragment || <div />; <__f>${code}</__f>;`;
+const wrapCodeInFragment = code =>
+	`const Fragment__ = React.Fragment || 'div'; <Fragment__>${code}</Fragment__>;`;
 
 // Wrap everything in a React component to leverage the state management
 // of this component