Upgrade chai to 4

package.json

@@ -36,7 +36,7 @@
     "vtfeatures": "node bin/extract_vtfeatures.js src/**/*.ts src/*.ts"
   },
   "devDependencies": {
-    "@types/chai": "^3.4.34",
+    "@types/chai": "^4.2.11",
     "@types/debug": "^4.1.5",
     "@types/deep-equal": "^1.0.1",
     "@types/glob": "^5.0.35",
@@ -48,7 +48,7 @@
     "@types/ws": "^7.2.3",
     "@typescript-eslint/eslint-plugin": "^2.25.0",
     "@typescript-eslint/parser": "^2.25.0",
-    "chai": "3.5.0",
+    "chai": "^4.2.0",
     "deep-equal": "^2.0.1",
     "eslint": "^6.8.0",
     "express": "^4.17.1",