fix travis build

[ltalirz]

ran into this issue travis-ci/travis-ci#8536 in pr #75 (see also #75 (comment))

fixed by using more than one test job (now testing on python 3.7 and python 3.8).

[codecov]

Codecov Report

Merging #77 into master will increase coverage by 0.42%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #77      +/-   ##
==========================================
+ Coverage   81.81%   82.24%   +0.42%     
==========================================
  Files          33       33              
  Lines        1628     1667      +39     
==========================================
+ Hits         1332     1371      +39     
  Misses        296      296

Impacted Files	Coverage Δ
...imade/filtertransformers/tests/test_transformer.py	100% <0%> (ø)	⬆️
optimade/filterparser/tests/test_filterparser.py	100% <0%> (ø)	⬆️
optimade/models/toplevel.py	100% <0%> (ø)	⬆️
optimade/models/references.py	95.12% <0%> (+0.12%)	⬆️
optimade/server/main.py	66.66% <0%> (+0.23%)	⬆️
optimade/models/baseinfo.py	92.59% <0%> (+0.28%)	⬆️
optimade/server/mappers/structures.py	94.28% <0%> (+0.34%)	⬆️
optimade/models/links.py	90.9% <0%> (+0.43%)	⬆️
optimade/models/jsonapi.py	88.15% <0%> (+0.48%)	⬆️
optimade/filtertransformers/tests/test_django.py	88.88% <0%> (+0.65%)	⬆️
... and 3 more

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[ml-evs]

Thanks for sorting this @ltalirz!