Add babel-runtime dependency required by ethjs-query

The version of `ethjs-query` we're using has an unlisted dependency
upon `babel-runtime`, and will crash if it's not present. Currently
it's working because `babel-runtime` is being brought in by another
dependency (`fetch-mock`), but that dependency is removed in another
PR (#340).

Until we can migrate away from using this version of `ethjs-query`,
`babel-runtime` has been added as an explicit dependency to ensure it
is present.

`ethjs-query` is a transitive dependency of `eth-method-registry` via
`ethjs`. There is a PR open on `eth-method-registry` to replace `ethjs`
altogether [1]. Once those are merged and released, we can update
`eth-method-registry` here and remove this `babel-runtime` dependency.

package.json

@@ -37,6 +37,7 @@
   "dependencies": {
     "@metamask/contract-metadata": "^1.22.0",
     "async-mutex": "^0.2.6",
+    "babel-runtime": "^6.26.0",
     "eth-ens-namehash": "^2.0.8",
     "eth-json-rpc-infura": "^5.1.0",
     "eth-keyring-controller": "^6.1.0",