chore: pin @types/chai to 4.0.6 (#412)

Author: felixfbecker

package.json

@@ -62,7 +62,7 @@
     "@sourcegraph/prettierrc": "^2.0.0",
     "@sourcegraph/tsconfig": "^1.0.0",
     "@sourcegraph/tslint-config": "^9.0.2",
-    "@types/chai": "^4.0.0",
+    "@types/chai": "4.0.6",
     "@types/chai-as-promised": "^7.1.0",
     "@types/glob": "^5.0.30",
     "@types/lodash": "^4.14.76",