Travis: add coverage reporting via pytest-cov on codecov.io

[blueyed]

Can be viewed here: https://codecov.io/gh/pytest-dev/pytest-django/tree/coverage

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@6c5d3fb). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #520   +/-   ##
=========================================
  Coverage          ?   85.72%           
=========================================
  Files             ?       32           
  Lines             ?     1611           
  Branches          ?      139           
=========================================
  Hits              ?     1381           
  Misses            ?      177           
  Partials          ?       53

Flag	Coverage Δ
#django110	81.81% <ø> (?)
#django111	83.55% <ø> (?)
#django17	79.63% <ø> (?)
#django18	80.81% <ø> (?)
#django19	79.88% <ø> (?)
#mysql_innodb	79.08% <ø> (?)
#mysql_myisam	79.01% <ø> (?)
#postgres	81.19% <ø> (?)
#py27	82.99% <ø> (?)
#py34	80.88% <ø> (?)
#py35	79.2% <ø> (?)
#py36	80.88% <ø> (?)
#pypy	81.56% <ø> (?)
#pypy3	80.75% <ø> (?)
#pytest3	85.72% <ø> (?)
#sqlite	81.56% <ø> (?)
#sqlite_file	80.38% <ø> (?)

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[blueyed]

Should be renamed (see pytest-dev/pytest#2800 (comment)).