Start of adding tests for django 2.2

[streeter]

What's this PR do?

Starts testing pinax-stripe against Django 2.2 (and Django 2.1 explicitly)

Any background context you want to provide?

The test configuration was not testing against the most recent version of Django.

What ticket or issue # does this fix?

Definition of Done (check if considered and/or addressed):

• [x] Are all backwards incompatible changes documented in this PR?
• [x] Have all new dependencies been documented in this PR?
• [x] Has the appropriate documentation been updated (if applicable)?
• [x] Have you written tests to prove this change works (if applicable)?

[codecov[bot]]

Codecov Report

Merging #631 into master will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #631   +/-   ##
=======================================
  Coverage   99.21%   99.21%           
=======================================
  Files          33       33           
  Lines        1911     1911           
  Branches      175      175           
=======================================
  Hits         1896     1896           
  Misses          7        7           
  Partials        8        8

Flag	Coverage Δ
#py27dj110	98.9% <ø> (ø)	:arrow_up:
#py27dj111	98.9% <ø> (ø)	:arrow_up:
#py27dj18	99.16% <ø> (ø)	:arrow_up:
#py34dj110	98.9% <ø> (ø)	:arrow_up:
#py34dj111	98.9% <ø> (ø)	:arrow_up:
#py34dj18	99.16% <ø> (ø)	:arrow_up:
#py34dj20	98.9% <ø> (ø)	:arrow_up:
#py35dj110	98.9% <ø> (ø)	:arrow_up:
#py35dj111	98.9% <ø> (ø)	:arrow_up:
#py35dj18	99.16% <ø> (ø)	:arrow_up:
#py35dj20	98.9% <ø> (ø)	:arrow_up:
#py36dj111	98.9% <ø> (ø)	:arrow_up:
#py36dj20	98.9% <ø> (ø)	:arrow_up:
#py36dj20psql	98.9% <ø> (ø)	:arrow_up:

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