fix: add Promise return type for proxy function (#332)

Compiling this project in TypeScript gave the following error:
```
error TS2314: Generic type 'Promise<T>' requires 1 type argument(s).
declare function proxy(proxyParams: ProxyParams): Promise
```
This commit tries to fix this error without being explicit with the return type.

src/proxy.d.ts

@@ -5,6 +5,6 @@ interface ProxyParams {
   binaryMimeTypes?: string[]
 }
 
-declare function proxy(proxyParams: ProxyParams): Promise
+declare function proxy(proxyParams: ProxyParams): Promise<any>
 
 export default ProxyParams