Fix Django 2 compatibility

[thomwiggers]

Try to import reverse from django.urls

[codecov-io]

Codecov Report

Merging #20 into develop will increase coverage by 0.22%. The diff coverage is 100%.

@@             Coverage Diff             @@
##           develop      #20      +/-   ##
===========================================
+ Coverage    91.74%   91.96%   +0.22%     
===========================================
  Files            6        6              
  Lines          218      224       +6     
===========================================
+ Hits           200      206       +6     
  Misses          18       18

Impacted Files	Coverage Δ
tinymce/views.py	86.88% <100%> (+0.67%)	:arrow_up:
tinymce/widgets.py	91.81% <100%> (+0.22%)	:arrow_up:

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[romanvm]

Thanks! However I would ask you to do 2 things: 1) please submit your PR against develop branch, 2) please add Django 2 to the Travis CI build matrix to make sure that all test suite is passed.

[thomwiggers]

So filebrowser appears to be affected by the same problem. I'll see if I can fix it there as well.

[thomwiggers]

So django-filebrowser-no-grappelli appears to be affected more, I'll work on it later.

[romanvm]

Thank anyway. However, even after removing filebrowser form the test application I haven't been able to run the test suite locally because of broken access to Django admin that works perfectly in 1.11. Django 2.0 is still in early alpha so I wouldn't rush with it.

[romanvm]

I've added a workaround that allows testing in Django 2.0 without filebrowser. So now it's save to merge your PR. Thanks!