Use batch_alter_table to drop column in the migration script

[x4base]

Should we use render_as_batch=True in context.configure in migrations/env.py as well?

[landscape-bot]

Code quality remained the same when pulling cb3a531 on x4base:batch_alter_table into bd47a29 on airbnb:master.

[coveralls]

Coverage remained the same at 80.986% when pulling cb3a531 on x4base:batch_alter_table into bd47a29 on airbnb:master.

[mistercrunch]

I don't feel strongly about that, but we may want to have a unit test that tests downgrades. We probably want to also setup travis builds on different databases (sqlite, mysql, postgres) eventually.

[mistercrunch]

I read the docs about render_as_batch here:
http://alembic.readthedocs.io/en/latest/batch.html
and it seems reasonable, I'd approve in a PR that adds downgrading tests that go all the way, provided that this new test also works on my local MySQL.

[x4base]

So I should add the downgrading tests here, right?

[mistercrunch]

This is fine as is, I'll merge.

In the future we should probably test downgrades on multiple databases in Travis.