Reports: fix an SQL query for expected_speakers and non_attending_speakers
Reports: fix an SQL query for expected_speakers and non_attending_speakers

Fixes the following error on a production system running PostgreSQL:

An ActiveRecord::StatementInvalid occurred in reports#show_people:

  PG::GroupingError: ERROR:  column "events.start_time" must appear in the GROUP BY clause or be used in an aggregate function
LINE 1: ...d', 'scheduled') GROUP BY "people"."id"  ORDER BY events.sta...
                                                             ^
: SELECT COUNT(*) AS count_all, "people"."id" AS people_id FROM "people" INNER JOIN "event_people" ON "event_people"."person_id" = "people"."id" INNER JOIN "events" ON "events"."id" = "event_people"."event_id" INNER JOIN "conferences" ON "conferences"."id" = "events"."conference_id" WHERE "conferences"."id" = 7 AND "event_people"."event_role" IN ('speaker', 'moderator') AND "event_people"."role_state" = 'confirmed' AND "events"."public" = 't' AND (events.start_time > '2017-06-08 13:07:51.260259') AND (events.start_time < '2017-06-08 17:07:51.260380') AND "events"."state" IN ('unconfirmed', 'confirmed', 'scheduled') GROUP BY "people"."id"  ORDER BY events.start_time ASC
  app/controllers/reports_controller.rb:121:in `show_people'

fossgis/frab,fossgis/frab,fossgis/frab,fossgis/frab