clean up unix and drone

two ideas to simplify this:

1. simpler apt-get

    ```yaml
    - sudo apt-get install mongodb-clients
    - mongo --eval 'rs.initiate()'
    ```

2. no apt-get: run `rs.initiate()` from the test code via [Run](http://godoc.org/gopkg.in/mgo.v2#Database.Run) and the [`replSetInitiate`](http://docs.mongodb.org/manual/reference/command/replSetInitiate/) db command