Updated py-vdirsyncer to 0.13.1.

Version 0.13.1
==============

*released on 30 September 2016*

- Fix a bug that would completely break collection discovery.

Version 0.13.0
==============

*released on 29 September 2016*

- Python 2 is no longer supported at all. See :gh:`219`.
- Config sections are now checked for duplicate names. This also means that you
  cannot have a storage section ``[storage foo]`` and a pair ``[pair foo]`` in
  your config, they have to have different names. This is done such that
  console output is always unambigous. See :gh:`459`.
- Custom commands can now be used for conflict resolution during sync. See
  :gh:`127`.
- :storage:`http` now completely ignores UIDs. This avoids a lot of unnecessary
  down- and uploads.

(wiz)