Update py-vdirsyncer to 0.7.4:

Version 0.7.4
=============

*released on 22 December 2015*

- Improved error messages instead of faulty server behavior, see :gh:`290` and
  :gh:`300`.
- Safer shutdown of threadpool, avoid exceptions, see :gh:`291`.
- Fix a sync bug for read-only storages see commmit
  `ed22764921b2e5bf6a934cf14aa9c5fede804d8e`.
- Etag changes are no longer sufficient to trigger sync operations. An actual
  content change is also necessary. See :gh:`257`.
- :storage:`remotestorage` now automatically opens authentication dialogs in
  your configured GUI browser.
- **Packagers:** ``lxml>=3.1`` is now required (newer lower-bound version).

(wiz)