Replace babel-preset-es2015 with babel-preset-env

This adds support for transpiling newer JS language features (ES2016+) and will
make it easier to update going forwards.

Decisions over what features to transpile for backwards compatibility
are controlled by browser coverage queries. See the browserslist docs [1] for
details of the query format.

[1] https://github.com/browserslist/browserslist

.babelrc

 {
-  "presets": ["es2015"],
+  "presets": [
+    ["env", {
+       "targets": {
+         "browsers": ["> 0.3%"]
+      }
+    }]
+  ],
   "ignore": "**/vendor/*"
 }

package.json

@@ -17,7 +17,7 @@
     "autofill-event": "0.0.1",
     "autoprefixer": "^9.4.7",
     "aws-sdk": "^2.345.0",
-    "babel-preset-es2015": "^6.24.0",
+    "babel-preset-env": "^1.7.0",
     "babelify": "^7.3.0",
     "browserify": "^16.2.3",
     "browserify-istanbul": "^2.0.0",