fix: package.json to reduce vulnerabilities

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/SNYK-JS-FORMDATA-10841150

Author: snyk-bot

package.json

@@ -16,7 +16,7 @@
     "dirty-chai": "^2.0.1",
     "eslint": "^6.8.0",
     "istanbul": "^1.0.0-alpha.2",
-    "lerna": "^3.19.0",
+    "lerna": "^6.5.0",
     "mocha": "^6.2.2",
     "mocha-clean": "^1.0.0",
     "mocha-sinon": "^2.0.0",