Fix deployment heroku

This pull request makes changes to allow for successful out-of-the-box deployment to Heroku. The changes are:

- Generate binstubs and add them to repo (also removing bin/ from .gitignore)

If there are good reasons to ignore the binstubs, then an alternative (untested) would be adding a statement to the README specifying that the user run `rake rails:update:bin` to generate these files on the server. The problem is, ironically, I'm not sure you'll have access to the rake bin on the server if you haven't included the binstubs in the repo. :smiley: 

EDIT 6/12 6:18 PM ET: It looks like another alternate approach is to specify to run `bundle install --binstubs`; again unclear how this works on Heroku, though. I think it would locally from vendor/binstubs, and so would still need to be added to the repo.

- Rename `:heroku` group to `:production` in Gemfile

I understand this change may reduce redeployability on other platforms (very slightly; just adds an unused gem in that case, as far as I can tell). But if you want to keep it named `:heroku`, then other settings should be configured so that a push "just works" on Heroku (something it does not do currently). This change resolves assets not being served correctly.

- In README deploy section, change `Organization.first.owners_team << User.first` to `Organization.first.owners_team.users << User.first`

The first statement simply doesn't work; I think the latter is what was meant (since it appears to work), but please edit if not.

IMPORTANT NOTE: If accepting this pull request, binstubs will be added. If rejecting that approach, please accept my other pull request ( https://github.com/dobtco/procure-io/pull/193 ) which adds the binstub note to the README (please do not accept both).

Closes #192 -- though not the broader question of CI for keeping deployment instructions up-to-date.

cc @migurski FYI