Fix a small ImapClient bug

Summary: I found a small bug in ImapClient which makes it crash when making a SEARCH using MODSEQ. The bug will be fixed in 0.12. In the meantime, I changed requirements.txt to point to our ImapClient fork.

Test Plan: Ran a full sync and an update afterwards.

Reviewers: emfree, spang

Reviewed By: spang

Differential Revision: https://review.inboxapp.com/D315