Linkify: true doesn't work for React 16.4.1

There is an error thrown when linkify is set to true:

```react-dom.development.js?9cad:16543 Uncaught TypeError: Cannot read property 'getType' of undefined
at Autolinker.replaceFn (linkify.js?5214:33)
at Autolinker.createMatchReturnVal (Autolinker.js?287d:828)
at Autolinker.link (Autolinker.js?287d:802)
at Array.linkify (linkify.js?5214:109)
at Core.process (parser_core.js?12d2:50)
at Remarkable.parse (index.js?2259:138)
at Remarkable.render (index.js?2259:152)
at Remarkable.renderMarkdown (index.js?1e4d:76)
at Remarkable.content (index.js?1e4d:58)
at Remarkable.render (index.js?1e4d:42)```