Fix Flow type default export (#99)

The actual JS module only has a default export of an object with `createResizedImage` function.

This PR update the Flow definition to match the usage of the module. This fixes the following flow error:

```
 18: import ImageResizer from 'react-native-image-resizer';
            ^^^^^^^^^^^^ Default import from `react-native-image-resizer`. This module has no default export.
```

index.js.flow

@@ -6,7 +6,7 @@ declare type ResizedImageInfo = {
     name?: string
 };
 
-declare export function createResizedImage(
+declare function createResizedImage(
     uri: string,
     width: number,
     height: number,
@@ -16,3 +16,6 @@ declare export function createResizedImage(
     outputPath?: string
 ): Promise<ResizedImageInfo>;
 
+declare export default {
+  createResizedImage: createResizedImage
+};