fix(deps): manually pull in newer node-fetch to avoid babel prod dep

The published node-fetch@2.0.0-alpha.3 still has a runtime dependency
on babel-runtime, which *massively* increases the make-fetch-happen
install size.

I've put together a custom node-fetch package with at least one of my
(submitted) patches. It'll probably be this way until node-fetch
releases a full 2.0.0 version and starts keeping up with a release
cadence. Then, we can remove this local package and have a normal dep :)

package.json

@@ -5,7 +5,8 @@
   "main": "index.js",
   "files": [
     "*.js",
-    "lib"
+    "lib",
+    "node-fetch-pkg.tgz"
   ],
   "scripts": {
     "prerelease": "npm t",
@@ -39,7 +40,7 @@
     "checksum-stream": "^1.0.2",
     "lru-cache": "^4.0.2",
     "mississippi": "^1.2.0",
-    "node-fetch": "^2.0.0-alpha.3",
+    "node-fetch": "file:node-fetch-pkg.tgz",
     "promise-retry": "^1.1.1",
     "proxy-agent": "^2.0.0",
     "safe-buffer": "^5.0.1",