Add an end-to-end component test for App

Summary:
This should ensure that there aren’t any runtime PropTypes errors.

The change to reimplement `Object.entries` is due to the fact that my
local Node environment (v6.11.1) does not yet support that ESnext
function.

Test Plan:
Apply the following diff (`git apply`):
```diff
diff --git a/explorer/src/UserExplorer.js b/explorer/src/UserExplorer.js
index 8da252d..cdc1370 100644
--- a/explorer/src/UserExplorer.js
+++ b/explorer/src/UserExplorer.js
@@ -10,7 +10,7 @@ export class UserExplorer extends Component {
   static propTypes = {
     selectedPath: PropTypes.string.isRequired,
     selectedUser: PropTypes.string,
-    onSelectUser: PropTypes.func.isRequired,
+    onSelectUser: PropTypes.number.isRequired,
     data: commitUtilsPropTypes.commitData.isRequired,
   }
```
Then, `yarn test` should fail. Revert the diff, and `yarn test` should
pass.

wchargin-branch: react-proptypes-test

explorer/src/App.test.js

@@ -0,0 +1,10 @@
+import React from 'react';
+import ReactDOM from 'react-dom';
+import App from './App';
+
+// Check that PropTypes check out.
+it('renders without crashing', () => {
+  const div = document.createElement('div');
+  ReactDOM.render(<App />, div);
+  ReactDOM.unmountComponentAtNode(div);
+});

explorer/src/UserExplorer.js

@@ -16,7 +16,10 @@ export class UserExplorer extends Component {
 
   render() {
     const weights = userWeightForPath(this.props.selectedPath, this.props.data, commitWeight);
-    const sortedUserWeightTuples = Object.entries(weights).sort((a,b) => b[1] - a[1]);
+    const sortedUserWeightTuples =
+        Object.keys(weights)
+        .map(k => [k, weights[k]])
+        .sort((a,b) => b[1] - a[1]);
     const entries = sortedUserWeightTuples.map(authorWeight => { 
       const [author, weight] = authorWeight;
       return <UserEntry userId={author} weight={weight} key={author}/>