Django-2.2 support

[tumb1er]

• Updated setup.py and test-requires
• Added Django-2.2 to test matrix
• Removed "NotImplemented" from set_autocommit for sphinx: autocommit mode is supported since 1.x but can be used only with RT indices.

[codecov]

Codecov Report

Merging #15 into master will increase coverage by 0.08%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #15      +/-   ##
==========================================
+ Coverage   90.93%   91.02%   +0.08%     
==========================================
  Files          14       14              
  Lines        1225     1225              
==========================================
+ Hits         1114     1115       +1     
+ Misses        111      110       -1

Impacted Files	Coverage Δ
sphinxsearch/backend/sphinx/base.py	84.61% <ø> (+0.65%)	⬆️
testproject/testapp/tests.py	99.52% <100%> (ø)	⬆️
testproject/settings.py	100% <100%> (ø)	⬆️

---

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Δ = absolute <relative> (impact), ø = not affected, ? = missing data
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[codecov]

Codecov Report

Merging #15 into master will increase coverage by 0.08%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #15      +/-   ##
==========================================
+ Coverage   90.93%   91.02%   +0.08%     
==========================================
  Files          14       14              
  Lines        1225     1225              
==========================================
+ Hits         1114     1115       +1     
+ Misses        111      110       -1

Impacted Files	Coverage Δ
sphinxsearch/backend/sphinx/base.py	84.61% <ø> (+0.65%)	⬆️
testproject/testapp/tests.py	99.52% <100%> (ø)	⬆️
testproject/settings.py	100% <100%> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 6a3171a...a0c52ed. Read the comment docs.