build: replace babel-preset-es2015 with babel-preset-env (#1027)

The Babel team has deprecated `babel-preset-es2015` and suggests we use `babel-preset-env` instead. This suppresses the following warning:

```
npm WARN deprecated babel-preset-es2015@6.24.1: 🙌  Thanks for using Babel: we recommend using babel-preset-env now: please read babeljs.io/env to update!
```

As suggested, see [babeljs.io/env](http://babeljs.io/docs/en/env/) for more.

.babelrc

@@ -1,6 +1,6 @@
 {
   "presets": [
-    "es2015"
+    "env"
   ],
   "plugins": [
     "transform-object-rest-spread"

package.json

@@ -55,7 +55,7 @@
     }
   },
   "scripts": {
-		"api-docs": "jsdoc --configure .jsdoc.json",
+    "api-docs": "jsdoc --configure .jsdoc.json",
     "build": "grunt",
     "test-dts": "tsc",
     "test": "npm run test-dts && grunt test",
@@ -75,7 +75,7 @@
     "babel-core": "^6.26.0",
     "babel-plugin-transform-object-rest-spread": "^6.6.5",
     "babel-polyfill": "^6.7.4",
-    "babel-preset-es2015": "^6.6.0",
+    "babel-preset-env": "^1.7.0",
     "babelify": "^8.0.0",
     "blanket": "~1.2.3",
     "chai": "~4.1.2",