NO-JIRA: enable code coverage report in travis

[kgiusti]

This patch enables code coverage reporting on travis.

If the build + tests are successful, at the end of the output log a url will be printed. That url can be used to view the coverage report. Example:

==> Uploading reports
url: https://codecov.io
query: branch=travis-coverage&commit=99e369453e37f96268e8b08b340b78dd616c2673&build=42.1&build_url=&name=&tag=&slug=kgiusti%2Fdispatch&service=travis&flags=&pr=false&job=391325606
-> Pinging Codecov
-> Uploading
-> View reports at https://codecov.io/github/kgiusti/dispatch/commit/99e369453e37f96268e8b08b340b78dd616c2673

Open the "View reports...." link in a browser, and click on the Files header for the coverage results.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@13c2107). Click here to learn what that means.
The diff coverage is n/a.

@@           Coverage Diff            @@
##             master    #321   +/-   ##
========================================
  Coverage          ?   86.5%           
========================================
  Files             ?      69           
  Lines             ?   15466           
  Branches          ?       0           
========================================
  Hits              ?   13379           
  Misses            ?    2087           
  Partials          ?       0

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