Turn off the bucket check-and-creation for the test environment

Checking to see that the bucket for the test environment exists, then creating it if not, is unnecessary and can actually cause problems.

Tests should not be relying on external resources, so explicitly using a test bucket is frowned upon; we shouldn't facilitate this behavior by creating the bucket. Additionally, some testing tools (Webmock, in particular) will (rightfully) hoot and holler about the external GET request in `AWS::S3::Bucket#exists?`, causing `rake test` and `rake db:migrate` to fail.

In sum, the check-and-create approach, when applied to the test environment, (A) encourages bad behavior and (B) actually makes the build process harder.