Use a gender-neutral pronoun when announcing rating

This changes the hard-coded "He" in "He rated it a 4" at https://github.com/nicksergeant/slacktappd/blob/master/index.js#L22

...that seems like the simplest solution to incorrectly saying "he" when a "she" drinks a beer. That is, until Untappd's API  gender to the [/user/info](https://untappd.com/api/docs#user_info) endpoint.