Add missing dependency

[ocelotl]

Without this, tox -e lint fails in my computer.

Fixes #613

[toumorokoshi]

@ocelotl maybe this is already clear to you, but it's probably happening because:

1. some packages depend on the newest dev version of dbapi, which isn't published yet
2. when a package in a virtualenv is installed, it will attempt to install those dependencies, which is searches on pypi

Installing the local package ensures that 0.7 is installed, so pip sees that and skips trying to resolve against pypi (when it won't find the version).

[toumorokoshi]

I think tox continues to become a larger, harder-to-untangle file, but probably should address that at a different time.