Travis: upgrade tox to 2.9.1, improve cache handling

[blueyed]

No description provided.

[codecov-io]

Codecov Report

Merging #529 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master    #529   +/-   ##
======================================
  Coverage    85.8%   85.8%           
======================================
  Files          32      32           
  Lines        1620    1620           
  Branches      140     140           
======================================
  Hits         1390    1390           
  Misses        177     177           
  Partials       53      53

Flag	Coverage Δ
#django110	81.79% <ø> (ø)	⬆️
#django111	83.51% <ø> (ø)	⬆️
#django17	79.69% <ø> (ø)	⬆️
#django18	80.86% <ø> (ø)	⬆️
#django19	79.93% <ø> (ø)	⬆️
#mysql_innodb	79.07% <ø> (ø)	⬆️
#mysql_myisam	79.01% <ø> (ø)	⬆️
#postgres	81.29% <ø> (ø)	⬆️
#py27	83.08% <ø> (ø)	⬆️
#py34	80.98% <ø> (ø)	⬆️
#py35	79.19% <ø> (ø)	⬆️
#py36	80.98% <ø> (ø)	⬆️
#pypy	81.66% <ø> (ø)	⬆️
#pypy3	80.86% <ø> (ø)	⬆️
#pytest3	85.8% <ø> (ø)	⬆️
#sqlite	81.66% <ø> (ø)	⬆️
#sqlite_file	80.37% <ø> (ø)	⬆️

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