Fix babel-preset-env implementation (#765)

* Add supported browsers list to babel-preset-env.

This was fixed in v7 of babel-preset-env so that you could use the
browserslist directly from package.json. We'll have to wait until Babel
v7 is release, see #702.

* add node 6 to babel-preset-env targets

* Only build for Node during testing.

.babelrc

@@ -1,7 +1,18 @@
 {
   "presets": [
     ["env", {
-      "modules": false
+      "modules": false,
+      "targets": {
+        "browsers": [
+          "last 2 Chrome versions",
+          "last 2 Opera versions",
+          "last 2 Firefox versions",
+          "last 2 Edge versions",
+          "last 2 Safari versions",
+          "last 2 iOS versions",
+          "last 2 ChromeAndroid versions"
+        ]
+      }
     }],
     "react"
   ],
@@ -46,7 +57,13 @@
   ],
   "env": {
     "test": {
-      "plugins": ["transform-es2015-modules-commonjs"]
+      "presets": [
+        ["env", {
+          "targets": {
+            "node": "6"
+          }
+        }]
+      ]
     }
   }
 }