Switch from dnspython 1.12.0 to dnspython 1.11.1 .

Summary:
flanker 0.4.22 requires dnsq>=1.1.3. The latest version of dnsq
available is 1.1.3, which requires dnspython=1.11.1.
Wwe were installing dnspython 1.12.0 since
0e0f70614df2a3992e037e760a7097056ed17859 to avoid
https://github.com/rthalley/dnspython/issues/31 .
8a4b8c96eeffb950194c088df18ebe5437f2dcf2 of dnspython fixes the
aforementioned issue and is included in dnspython 1.11.1 so we safely
un-bump it.

Test Plan:
Create a new virtual machine from master (as is, before this fix), start
bin/inbox-start from /vagrant . /vagrant/inbox/util/startup.py should complain
indicating flanker 0.4.22 as the offending package (we should definitely
improve our error reporting on this kind of issues). bin/inbox-start won't
actually start.
Doing the same with this patch should solve the issue.

Reviewers: dlitz, emfree

Reviewed By: emfree

Differential Revision: https://review.inboxapp.com/D664