Django 2.0 compat fix

[tungd]

_format_value is now format_value. It actually is deprecated since 1.10. I still keep the _format_value for backward compatible, since I'm not sure about your policy regarding support older Django version. Please let me know if it is ok to remove I will update the PR.

[patrys]

I think we should do what we do with most of our software stack: support the latest version (2.0) and the last LTS (1.11).

[codecov-io]

Codecov Report

Merging #49 into master will increase coverage by 5.28%. The diff coverage is 100%.

@@           Coverage Diff           @@
##           master   #49      +/-   ##
=======================================
+ Coverage   88.71%   94%   +5.28%     
=======================================
  Files           5     3       -2     
  Lines         195   100      -95     
  Branches       24     9      -15     
=======================================
- Hits          173    94      -79     
+ Misses         12     4       -8     
+ Partials       10     2       -8

Impacted Files	Coverage Δ
django_prices/widgets.py	88.88% <100%> (ø)	:arrow_up:
django_prices/forms.py
django_prices/models.py

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[tungd]

If that's the case then keeping only format_value is fine, it's there since 1.11 (https://docs.djangoproject.com/en/1.11/releases/1.10/#id3). I've update the PR to reflect that.

Btw the test fail running Django 2.0 on Python 2.7, which is not supported.

[patrys]

@elwoodxblues @tungd I've fixed the tox config so it uses the correct Django versions.

[patrys]

Thank you! I'll release this with a minor version bump (as it affects the list of supported platforms).

[patrys]

Actually we already required 1.11+ so it'll be out as 0.7.1.