Fix field types in SubteamMembersChangedEvent

[youdofoo]

AddedUsersCount and RemovedUsersCount fields in SubteamMembersChangedEvent struct are defined as a string type, but in fact these fields in received data from subteam_members_changed have a number type.
So when you use slackevents.ParseEvent function to this event, following parse error occurs:

EventsAPI Error parsing inner event: unmarshalling_error, json: cannot unmarshal number into Go struct field SubteamMembersChangedEvent.added_users_count of type string: wrapError

To fix this, I changed the type of AddedUsersCount and RemovedUsersCount fields from string to int.

By the way, also in API document, "added_users_count" and "removed_users_count" are defined as a string.
I already reported this to https://slack.com/help.

[kanata2]

LGTM.
This PR breaks backward compatibility, so I'm planning to release in v0.11.0.