Fail a build on TravisCI when changes on models have missing migration files

[bugzilla-to-github]

This issue was created automatically by a script.

Bug 1677182

Bug Reporter: @jotes

Sometimes, It's easy to forget about generating a migration when the changes on a model don't cause visible side-effects (e.g. crashing the backend).

It's preventable; AFAIK Django's makemigrations has a --check flag to return a non-zero status when it finds model changes without a migration.