Deprecation warnings with Dateutil 2.5

[jenshnielsen]

After the release of Dateutil 2.5 I am seeing a number of Deprecation warnings when running the test suite:

It's not a big issue at the moment but we should preferably fix this before 2.0 so we remain compatible with future versions of Dateutil.

/Users/jhn/Envs/mplmaster/lib/python3.5/site-packages/dateutil/rrule.py:415: DeprecationWarning: Using both 'count' and 'until' is inconsistent with RFC 2445 and has been deprecated in dateutil. Future versions will raise an error.

[pganssle]

This line is likely the culprit. I think probably the best solution is to incorporate the old behavior into the rrulewrapper.

[tacaswell]

Can we just drop the use of count there?

[jenshnielsen]

@tacaswell I thinks so. The code clearly already takes into account the fact that the number of ticks may be larger than MAXTICKS and thus larger than count

[jenshnielsen]

fixed by #6190