Fix createClient to take PromiseConstructor (#1)

Author: unional

index.d.ts

@@ -17,7 +17,7 @@ export interface CreateClientOptions {
   retryOptions?: {
     interval?: number
   };
-  Promise?: Promise<any>;
+  Promise?: PromiseConstructor;
 }
 
 export interface RequestHandle {

spec/createClient.spec.ts

@@ -0,0 +1,11 @@
+import test from 'ava';
+
+import gmaps = require('@google/maps');
+
+test('with promise', t => {
+  gmaps.createClient({
+    key: '',
+    Promise
+  });
+  t.pass();
+});