chore: pin chai to 4.1.0

Author: greenkeeper[bot]

package.json

@@ -29,7 +29,7 @@
   },
   "homepage": "https://github.com/SimplePEG/JavaScript#readme",
   "devDependencies": {
-    "chai": "^4.1.0",
+    "chai": "4.1.0",
     "coveralls": "^2.11.9",
     "eslint": "^4.3.0",
     "istanbul": "^0.4.4",