Add is-plain-obj dependency

Author: ehmicky

package-lock.json

@@ -27,6 +27,7 @@
     "eslint-plugin-prettier": "^3.1.0",
     "execa": "^2.0.4",
     "gh-release": "^3.5.0",
+    "is-plain-obj": "^2.1.0",
     "make-dir": "^3.0.0",
     "npm-run-all": "^4.1.5",
     "prettier": "^1.18.2",