Add markdown dependency for testing

[carltongibson]

Ref https://travis-ci.org/carltongibson/django-filter/jobs/208346670#L1428

Merging #655 broke the build, failing a markdown import for latest DRF.

django.template.library.InvalidTemplateLibrary: Invalid template library specified. ImportError raised when trying to load 'rest_framework.templatetags.rest_framework': No module named 'markdown'

This adds the dependency.

[codecov-io]

Codecov Report

Merging #658 into develop will increase coverage by 0.09%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           develop     #658      +/-   ##
===========================================
+ Coverage    97.67%   97.77%   +0.09%     
===========================================
  Files           15       15              
  Lines         1077     1077              
===========================================
+ Hits          1052     1053       +1     
+ Misses          25       24       -1

Impacted Files	Coverage Δ
django_filters/widgets.py	100% <0%> (+0.78%)	✅

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