Update dependency portfinder to v1.0.21 (#476)

Author: renovate[bot]

package-lock.json

@@ -47,7 +47,7 @@
     "istanbul": "0.4.5",
     "mkdirp": "0.5.1",
     "mocha": "6.1.4",
-    "portfinder": "1.0.20",
+    "portfinder": "1.0.21",
     "selenium-assistant": "5.4.0",
     "semver": "6.2.0",
     "sinon": "7.3.2",