Vagrantfile: Fix `config.vm` -> `override.vm` :-(

Summary:
Oops.  This fixes the following error:

    The checksum of the dowloaded box did not match the expected
    value. Please verify that you have the proper URL setup and that
    you're downloading the proper file.

    Expected: b79e900774b6a27500243d28bd9b1770e428faa3d8a3e45997f2a939b2b63570
    Received: 9a8bdea70e1d35c1d7733f587c34af07491872f2832f0bc5f875b536520ec17e

Test Plan:
1. Wipe out my ~/.vagrant.d and my .vagrant directory
2. Run `vagrant up` on Linux (using the default virtualbox provider)

Reviewers: emfree

Reviewed By: emfree

Differential Revision: https://review.inboxapp.com/D381