Update to Mockttp v4

Author: pimterry

package.json

@@ -79,7 +79,7 @@
     "graphql-subscriptions": "^1.2.1",
     "graphql-tag": "^2.12.6",
     "lodash": "^4.17.21",
-    "mockttp": "^3.2.1",
+    "mockttp": "^4.0.0",
     "node-datachannel": "^0.12.0",
     "performance-now": "^2.1.0",
     "sdp-transform": "^2.14.1"