Added codecov

[ebreton]

• added pytest-django (to be able to run tests through pytest)
• added pytest-cov (to generate coverage report)
• added CODECOV_TOKEN in Travis (to push the results to codecov)

Finally !! it works !

Here is a summary of the different issues I had on the way:

• the tests needed to be run inside the container (because of requirements)
  • hence the docker-compose -f docker-compose-dev.yml exec web make test-travis in .travis.yml
• the tests needed to be run within the full composition (because of tests using DB and selenium)
  • hence the make build-travis in .travis.yml that does a docker-compose up -d
• codecov needs the git contex and can not be run inside our composition (eventhough it could run inside a container. But it fails with our composition since it does not take into account the environment variables that are defined at runtime as described in https://github.com/codecov/support/wiki/Testing-with-Docker. I have tried to docker-compose run web with the variable, but the run command does not properly link the created container with the other ones, wihch makes the tests fail)
  • hence the codecov in .travis.yml
• but codecov needs access to the coverage.xml file resulting from the tests
  • hence the new volume /usr/src/app/coverage.xml in Dockerfile and in docker-compose-dev.yml

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@31cc10e). Click here to learn what that means.
The diff coverage is 0%.

@@            Coverage Diff            @@
##             master      #68   +/-   ##
=========================================
  Coverage          ?   49.58%           
=========================================
  Files             ?       46           
  Lines             ?     1081           
  Branches          ?        0           
=========================================
  Hits              ?      536           
  Misses            ?      545           
  Partials          ?        0

Impacted Files	Coverage Δ
...oscience_exports/exports/pytests/test_fulltexts.py	100% <ø> (ø)
...nfoscience_exports/exports/pytests/test_authors.py	100% <ø> (ø)
infoscience_exports/exports/pytests/test_dates.py	100% <ø> (ø)
infoscience_exports/exports/versions.py	0% <0%> (ø)

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