Add Python 3.6, drop django 1.7, update Travis+Tox

[benjaoming]

Takes over from #41

[codecov-io]

Codecov Report

Merging #47 into master will increase coverage by <.01%. The diff coverage is 73.97%.

@@            Coverage Diff             @@
##           master      #47      +/-   ##
==========================================
+ Coverage   76.54%   76.55%   +<.01%     
==========================================
  Files          25       25              
  Lines         597      610      +13     
==========================================
+ Hits          457      467      +10     
- Misses        140      143       +3

Impacted Files	Coverage Δ
django_nyt/tests/test_management.py	100% <ø> (ø)	:arrow_up:
django_nyt/tests/__init__.py	100% <100%> (ø)	:arrow_up:
django_nyt/tests/test_basic.py	100% <100%> (ø)	:arrow_up:
django_nyt/tests/test_views.py	100% <100%> (ø)	:arrow_up:
django_nyt/management/commands/notifymail.py	70.8% <71.21%> (+2.25%)	:arrow_up:
django_nyt/urls.py	85.71% <0%> (-14.29%)	:arrow_down:

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[timeyyy]

cool :)