update to mobx-react v6, remove mobx-react-devtools as it is not compatible with v6

src/index.tsx

@@ -2,7 +2,6 @@ import * as React from 'react';
 import * as ReactDOM from 'react-dom';
 import {observable} from 'mobx';
 import {observer} from 'mobx-react';
-import DevTools from 'mobx-react-devtools';
 
 class AppState {
     @observable timer = 0;
@@ -26,7 +25,6 @@ class TimerView extends React.Component<{appState: AppState}, {}> {
                 <button onClick={this.onReset}>
                     Seconds passed: {this.props.appState.timer}
                 </button>
-                <DevTools />
             </div>
         );
      }