Convert all files to ES2015 (node-fetch#182)

Elements of this commit come from node-fetch#140 by @gwicke.

Author: TimothyGu

package.json

@@ -6,7 +6,7 @@
   "scripts": {
     "build": "babel -d lib src",
     "prepublish": "npm run build",
-    "test": "mocha --compilers js:babel-register test/test.js",
+    "test": "mocha --compilers js:babel-polyfill --compilers js:babel-register test/test.js",
     "report": "cross-env BABEL_ENV=test nyc --reporter lcov --reporter text mocha -R spec test/test.js",
     "coverage": "cross-env BABEL_ENV=test nyc --reporter lcovonly mocha -R spec test/test.js && cat ./coverage/lcov.info | coveralls"
   },
@@ -29,6 +29,7 @@
     "babel-cli": "^6.16.0",
     "babel-plugin-istanbul": "^2.0.1",
     "babel-plugin-transform-runtime": "^6.15.0",
+    "babel-polyfill": "^6.16.0",
     "babel-preset-es2015": "^6.16.0",
     "babel-register": "^6.16.3",
     "bluebird": "^3.3.4",
@@ -44,6 +45,7 @@
     "resumer": "0.0.0"
   },
   "dependencies": {
+    "babel-runtime": "^6.11.6",
     "encoding": "^0.1.11",
     "is-stream": "^1.0.1"
   },
@@ -67,6 +69,7 @@
       "src/*.js"
     ],
     "require": [
+      "babel-polyfill",
       "babel-register"
     ],
     "sourceMap": false,