Use tox to run test in travis

[sliverc]

This way we can harmonize tox and travis configuration

Fixes #391

Description of the Change

Checklist

• PR only contains one change (considered splitting up PR)
• unit-test added (not needed as only build change)
• documentation updated (not needed as only build change)
• changelog entry added to CHANGELOG.md
• author name in AUTHORS

[mblayman]

👍 Nice!

[codecov-io]

Codecov Report

Merging #440 into master will increase coverage by 0.65%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master    #440      +/-   ##
=========================================
+ Coverage   91.95%   92.6%   +0.65%     
=========================================
  Files          57      54       -3     
  Lines        2994    2812     -182     
=========================================
- Hits         2753    2604     -149     
+ Misses        241     208      -33

Impacted Files	Coverage Δ
example/api/serializers/__init__.py
example/migrations/__init__.py

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[sliverc]

@mblayman I would have been too lucky to get it right in the first run. Coverage was not working anymore and is now fixed. Could you have a look again? Thanks.

[mblayman]

Still looks good to me. Thanks for doing this one. I think it will help the tox file stay up-to-date. 👍