Fix error with underscore string

packages/rocketchat-lib/server/functions/checkUsernameAvailability.coffee

 RocketChat.checkUsernameAvailability = (username) ->
-	return not Meteor.users.findOne({ username: { $regex : new RegExp("^" + _.trim(username) + "$", "i") } })
\ No newline at end of file
+	return not Meteor.users.findOne({ username: { $regex : new RegExp("^" + s.trim(username) + "$", "i") } })
\ No newline at end of file

packages/rocketchat-lib/server/functions/setUsername.coffee

 RocketChat.setUsername = (user, username) ->
-	username = _.trim username
+	username = s.trim username
 	if not user or not username
 		return false