refactor(index.js): remove HeatmapLayer export

* Ref #470, #376, #375

BREAKING CHANGE: remove top-level HeatmapLayer export

Before:

```js
import { HeatmapLayer } from "react-google-maps"
```

After:

```js
import HeatmapLayer from "react-google-maps/lib/visualization/HeatmapLayer"
```

Author: tomchentw

src/lib/__tests__/index.test.js

@@ -4,7 +4,6 @@ import {
   GoogleMap,
   Circle,
   DirectionsRenderer,
-  HeatmapLayer,
   InfoWindow,
   KmlLayer,
   FusionTablesLayer,
@@ -22,7 +21,6 @@ describe(`index`, () => {
     expect(GoogleMap).toBeDefined();
     expect(Circle).toBeDefined();
     expect(DirectionsRenderer).toBeDefined();
-    expect(HeatmapLayer).toBeDefined();
     expect(InfoWindow).toBeDefined();
     expect(KmlLayer).toBeDefined();
     expect(FusionTablesLayer).toBeDefined();

src/lib/index.js

@@ -42,11 +42,6 @@ export {
   default as DirectionsRenderer,
 } from "./DirectionsRenderer";
 
-export {
-  // Provides backward compatibility. Will remove in 7.0.0
-  default as HeatmapLayer,
-} from "./visualization/HeatmapLayer";
-
 export {
   default as InfoWindow,
 } from "./InfoWindow";