Revert "Tell env preset to only use the polyfills we actually need"

This reverts commit 8ad2357.

Given we target node 4, this will include a *bunch* of polyfills anyway,
so we may as well just use babel-polyfill directly. This can be revised if
we decide to move away from node 4 support down the line.

.babelrc

@@ -1,8 +1,8 @@
 {
   "presets": [
     [ "env"
-    , { targets: { node: 4 }
-      , useBuiltIns: true
+    , { targets:
+        { node: 4 }
       }
     ]
   ],
@@ -11,4 +11,4 @@
       "plugins": [ "istanbul" ]
     }
   }
-}
+}