Update py-vdirsyncer to 0.9.3.

Version 0.9.3
=============

*released on 22 March 2016*

- :storage:`singlefile` and :storage:`http` now handle recurring events
  properly.
- Fix a typo in the packaging guidelines.
- Moved to ``pimutils`` organization on GitHub. Old links *should* redirect,
  but be aware of client software that doesn't properly handle redirects.

Version 0.9.2
=============

*released on 13 March 2016*

- Fixed testsuite for environments that don't have any web browser installed.
  See :ghpr:`384`.

Version 0.9.1
=============

*released on 13 March 2016*

- Removed leftover debug print statement in ``vdirsyncer discover``, see commit
  ``3d856749f37639821b148238ef35f1acba82db36``.

- ``metasync`` will now strip whitespace from the start and the end of the
  values. See :gh:`358`.

- New ``Packaging Guidelines`` have been added to the documentation.

(wiz)