Fix TravisCI setup

[raphael-riel]

This fixes TravisCI setup so the tests can run fail.
Coverall is now back in the Checks-loop.

[coveralls]

Coverage decreased (-0.3%) to 82.709% when pulling 9b3bb5c on raphael-riel:hotfix/tests-suite into aac755f on djangonauts:master.

[coveralls]

Coverage decreased (-0.3%) to 82.709% when pulling 5547beb on raphael-riel:hotfix/tests-suite into aac755f on djangonauts:master.

[coveralls]

Coverage decreased (-0.3%) to 82.709% when pulling 5547beb on raphael-riel:hotfix/tests-suite into aac755f on djangonauts:master.

[coveralls]

Coverage decreased (-0.3%) to 82.709% when pulling a763094 on raphael-riel:hotfix/tests-suite into aac755f on djangonauts:master.

[nemesifier]

@raphael-riel I can't find the time to keep maintaining this project alone, would you like to step up and help me?

[raphael-riel]

Hi @nemesisdesign, unfortunately, I won't be able to put any time on this project. Also, I'm using this library on a legacy project for whom it's planned to migrate django-hstore to the Postgres' built-in support of Django. The other to PRs I opened was easy-fixes I've seen while doing maintenance job on the said project. Rest assured I'll share here any work I may do for the library.