FIX rake's vlad interference

Having vlad in rake task causes [top level methods name conflicts](https://github.com/seattlerb/vlad/issues/21).

Symptom for this is this error :

    $ RAILS_ENV=test rake db:migrate
    rake aborted!
    host required

Since we use heroku, we don't need vlad anyway, which is a tool for
deployment on dedicated servers.

Removed it from dependencies.

Close #6 (travis problem was that one)