Fixed coverage/ directory included in local builds

- without this, the `coverage/` directory gets included in builds
  because `grunt-contrib-copy` doesn't operate on just the folder name
- this avoids us zipping up coverage with builds and helps bring CI and
  local builds closer together
- my commit here was incorrect: 815a2d0f7d

.npmignore

@@ -9,7 +9,7 @@
 .c8rc.json
 jsconfig.json
 changelog.md*
-coverage
+coverage/**
 docs/**
 _site/**
 content/adapters/**