Replace types string with str.

Uses command:

    ag -l 'type ([^:]+): string' | \
        xargs gsed -r -i.bak -e 's/type ([^:]+): string/type \1: str/g'

Note: [-r for gsed (GNU sed) is needed for group
matching](http://superuser.com/a/336819/125262).

google/cloud/vision/connection.py

@@ -29,7 +29,7 @@ class Connection(base_connection.JSONConnection):
     :type http: :class:`httplib2.Http` or class that defines ``request()``.
     :param http: (Optional) HTTP object to make requests.
 
-    :type api_base_url: string
+    :type api_base_url: str
     :param api_base_url: The base of the API call URL. Defaults to the value
                          :attr:`Connection.API_BASE_URL`.
     """