t: add Coveralls CI
t: use coverall for master
Revert "t: use coverall for master"

This reverts commit eda4e3f0b31150268e840bbe10addca4b66ba84f.
Revert "t: add Coveralls CI"

This reverts commit e44d58bd4eae93f982c782a4330ac3ef5daf231b.