pin some dev dependencies that seem to have made breaking changes

Author: michaelficarra

package.json

@@ -58,10 +58,10 @@
     "@rollup/plugin-json": "^4.0.2",
     "@rollup/plugin-node-resolve": "^7.1.3",
     "babel-plugin-transform-es2017-object-entries": "0.0.5",
-    "chai": "^4.2.0",
+    "chai": "4.2.0",
     "eslint": "^6.8.0",
     "esprima": "~4.0.1",
-    "mocha": "^7.1.1",
+    "mocha": "7.1.1",
     "nyc": "^15.0.1",
     "pegjs": "~0.10.0",
     "rollup": "^1.32.1",