chore: bumping yargs to v15

fixes #1673

- we no longer need to pin to yargs 14 for node 6 & 8 support

Author: ErisDS

package-lock.json

@@ -23,7 +23,7 @@
   "dependencies": {
     "neo-async": "^2.6.0",
     "source-map": "^0.6.1",
-    "yargs": "^14.2.3"
+    "yargs": "^15.3.1"
   },
   "optionalDependencies": {
     "uglify-js": "^3.1.4"