Switch to Knex-based migrations

[harpojaeger]

Ref MoveOnOrg/Spoke#683 & MoveOnOrg/Spoke#692. For now, we're concerned narrowly with swapping out the migration system, not with replacing the rethink-knex-adapter entirely (that's a separate PR, and probably more than one).

[bchrobot]

I think we should completely separate database migrations from the application runtime. It is the developer's responsibility to ensure the database is in a valid state before running the application. It is also desirable in a production deployment to have total control over changes to database schema and not worry about accidentally starting Spoke without the correct combination of environment variables to prevent database schema changes.

The Heroku Procfile has a release phase which can be used to run migrations as part of the deploy process.

[Frydafly]

Related issue: https://github.com/MoveOnOrg/Spoke/issues/281