Require specific heroku platform-api gem

[davidakachaos]

Transferred from: https://github.com/davidakachaos/workless_revived/issues/16

Just ran into an issue where I was getting app crashes until I manually ran bundle update platform-api (and bundle update heroics), because apparently I was on an old version of platform-api?

I think it might be a good idea to tie this gem to a specific version of platform-api that it has been tested with. 👍