fix: replace `minitest-wscolor` with `minitest-reporters`
improve: set fixtures that allows user to run tests from scratch
fix: downgrade sprockets

There is an exception raised when trying to precompile assets, which
will obviously make heroku deployment fails.

cf. https://github.com/sstephenson/sprockets/issues/566 for details and
following up.
Merge branch 'master' into fix/tests

Conflicts:
	Gemfile.lock
Merge pull request #5 from AdrienGiboire/fix/tests

fix: tests workflow