Use sudo to install pip package in Travis-CI

[JeanFred]

This has started failing with variations on:

OSError: [Errno 13] Permission denied: '/usr/local/lib/python2.7/dist-packages/pytz-2017.2.dist-info'

Executing under sudo is hardly a solution, but a suggested workaround
per travis-ci/travis-ci#1705
(and we are doing it already to install tox)

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@6a9eff8). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #572   +/-   ##
=========================================
  Coverage          ?   79.76%           
=========================================
  Files             ?       43           
  Lines             ?     2846           
  Branches          ?      174           
=========================================
  Hits              ?     2270           
  Misses            ?      517           
  Partials          ?       59

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