Refactored out manual fs traversal in coverage.js
Refactored out manual fs traversal in coverage.js

See issue #4335

codecombat/codecombat,javatlacati/codecombat,jeremiahyan/codecombat,javatlacati/codecombat,fabichoi/codecombat,fabichoi/codecombat,jeremiahyan/codecombat,javatlacati/codecombat,codecombat/codecombat,jeremiahyan/codecombat,kidaa/codecombat,javatlacati/codecombat,codecombat/codecombat,icodegame/codecombat,codecombat/codecombat,codecombat/codecombat,icodegame/codecombat,fabichoi/codecombat,icodegame/codecombat,kidaa/codecombat,jeremiahyan/codecombat,kidaa/codecombat,kidaa/codecombat,fabichoi/codecombat,icodegame/codecombat,javatlacati/codecombat