chore: Update Istanbul to NYC

Author: nschonni

.gitignore

@@ -11,4 +11,5 @@ test/lcov.info
 test/fixtures/watching-css-out-01
 test/fixtures/watching-css-out-02
 coverage
-package-lock.json
+package-lock.json
+.nyc_output

package.json

@@ -27,11 +27,11 @@
   },
   "gypfile": true,
   "scripts": {
-    "coverage": "node scripts/coverage.js",
+    "coverage": "nyc report --reporter=text-lcov | coveralls",
     "install": "node scripts/install.js",
     "postinstall": "node scripts/build.js",
     "lint": "node_modules/.bin/eslint bin/node-sass lib scripts test",
-    "test": "node_modules/.bin/mocha test/{*,**/**}.js",
+    "test": "nyc mocha test/{*,**/**}.js",
     "build": "node scripts/build.js --force",
     "prepublish": "not-in-install && node scripts/prepublish.js || in-install"
   },
@@ -77,6 +77,7 @@
     "mocha": "^6.2.2",
     "mocha-lcov-reporter": "^1.2.0",
     "node-gyp": "^6.0.1",
+    "nyc": "^14.1.1",
     "object-merge": "^2.5.1",
     "read-yaml": "^1.0.0",
     "rimraf": "^2.5.2",