Update dependency fibers to v5 (googleworkspace#306)

Co-authored-by: renovate[bot] <renovate[bot]@users.noreply.github.com>

Author: renovate[bot]

package-lock.json

@@ -19,7 +19,7 @@
     "del": "1.2.1",
     "eslint": "4.19.1",
     "eslint-config-google": "0.14.0",
-    "fibers": "4.0.2",
+    "fibers": "5.0.0",
     "gas-local": "1.3.1",
     "gulp": "4.0.2",
     "gulp-clean": "0.4.0",