Update sync-request from 4.1.0 to 6.0.0

Tested by ensuring that the release url on the `/docs/install` page is still generated correctly (sync-request is only used by the getLatestRelease in lib/utils, which is used on the /docs/install page to automatically link to the latest release from GitHub)

package.json

@@ -40,7 +40,7 @@
     "run-sequence": "^2.2.1",
     "standard": "^11.0.1",
     "supertest": "^3.0.0",
-    "sync-request": "^4.0.3",
+    "sync-request": "^6.0.0",
     "universal-analytics": "^0.4.16",
     "uuid": "^3.2.1"
   },