Django 2.0 support

[zvolsky]

Tox tests were successful. Please check following:

• tox.ini: I have changed tox environment from py 3.5 to 3.6, I have added environment for dj 1.11, dj 2.0.

• There is problem with earlier url_patterns.regex.pattern, used on more places in generator, test_command and test_generator. I think it could be replaced with str(url_patterns.pattern) in dj 2.0+. Temporary I have used common function get_pattern() which I suggest to use as long as single writing is not possible. But maybe you have better solution.

Best regards, Mirek

[codecov-io]

Codecov Report

Merging #1 into master will decrease coverage by 0.44%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master       #1      +/-   ##
==========================================
- Coverage   96.23%   95.79%   -0.45%     
==========================================
  Files           5        5              
  Lines         186      214      +28     
  Branches       24       30       +6     
==========================================
+ Hits          179      205      +26     
- Misses          6        7       +1     
- Partials        1        2       +1

Impacted Files	Coverage Δ
django_smoke_tests/generator.py	96.21% <100%> (+0.21%)	:arrow_up:
...ngo_smoke_tests/management/commands/smoke_tests.py	96.61% <0%> (-3.39%)	:arrow_down:

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[kamilkijak]

Mirek, you've done a great work. Thank you!

It took a little bit longer, because I wanted to test it against Django2 project and I didn't have any so far on my computer. I set up a simple project with few endpoints and it worked like a charm.

To make sure that everything is fine, I added two commits to this PR: 1) one that adds Django2 environment in Travis CI, 2) second one with a simple URL in Django2 style (path('articles/<int:year>/', views.year_archive)), which will be taken into account only for Django2 tests.