Add pre_checkout_query to PAYLOAD_TYPES (#238)

* Update Bot API version from 7.5 to 7.7
- Add new method: sendPaidMedia

* Add pre_checkout_query to PAYLOAD_TYPES

- Added pre_checkout_query to updates_controller.rb
- Updated tests in updates_controller_spec.rb
- Updated tests in typed_update_spec.rb
- Updated CHANGELOG.md for Bot API 7.7 and pre_checkout_query

CHANGELOG.md

@@ -1,5 +1,8 @@
 # Unreleased
 
+- Update to Bot API 7.7
+- Added `pre_checkout_query` to PAYLOAD_TYPES
+
 # 0.16.2
 
 - Update to Bot API 7.3