fix(api): check if release.build is NoneType

[bacongobbler]

closes #1077

[deis-bot]

@helgi, @kmala and @Joshua-Anderson are potential reviewers of this pull request based on my analysis of git blame information. Thanks @bacongobbler!

[helgi]

Just had a look, the reason why this fails is because the code assumes (incorrectly I suppose) that it's being called from within run/scale/deploy and all of those already do a release.build check and raise an exception. _gather_app_settings should not be even returning anything without release.build being set

#1076 made it so the release object does not have Build and is calling _gather_app_settings directly, whichi s why it got triggered.

I think you can overall change all the above checks to just this at the top of the function

        if release.build is None:
            raise DeisException('No build associated with this release to run this command')

[bacongobbler]

This also consistently fails because I'm on my laptop and don't have python3 installed while at the conference so I'm just abusing CI as my tester.

_gather_app_settings should not be even returning anything without release.build being set

Sure, I'll make that change.