build: fix lost is-promise

Author: Scrum

package.json

@@ -75,6 +75,7 @@
     "eslint-plugin-unicorn": "^4.0.2",
     "execa": "^0.9.0",
     "husky": "^0.14.3",
+    "is-promise": "^2.1.0",
     "lint-staged": "^6.1.1",
     "nyc": "^11.5.0",
     "posthtml-bem": "^0.2.2",