[ci] Implicitly get coverage version from coveralls

marfgold1 · web-flow · commit 457c6ebb181e · 2019-12-19T09:35:29.000+07:00
The coveralls package already has a dependency on coverage
and sets the boundary according to its support for the coverage library.
We should not duplicate this information.
diff --git a/requirements-test.txt b/requirements-test.txt
@@ -1,4 +1,3 @@
-coverage
 coveralls
 responses
 openwisp-utils[qa]>=0.2.1
