docs: update google maps code in readme (#1030)

* docs: update google maps code in readme

* fix: throw exception if API key is not provided

* fix: tests

* docs: update google maps docs

Author: atymic

Tests/ProviderAggregatorTest.php

@@ -13,7 +13,6 @@
 namespace Geocoder\Tests;
 
 use Geocoder\Collection;
-use Geocoder\Geocoder;
 use Geocoder\Model\Address;
 use Geocoder\Model\AddressCollection;
 use Geocoder\Query\GeocodeQuery;