Clear cache

[benjaoming]

This should fix #34 by always clearing the cache when something is created (or deleted). In this case, the rollback doesn't AFAIK trigger the post_delete signal, but during tests, the post_save signal should be triggered to clear the cache.

It could also prevent other sorts of odd problems!

[codecov-io]

Codecov Report

Merging #36 into master will increase coverage by 0.11%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #36      +/-   ##
==========================================
+ Coverage      76%   76.11%   +0.11%     
==========================================
  Files          25       25              
  Lines         600      603       +3     
==========================================
+ Hits          456      459       +3     
  Misses        144      144

Impacted Files	Coverage Δ
django_nyt/models.py	85.38% <100%> (+0.34%)	⬆️

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