Internal: upgrade to babel6

Upgrades to babel 6. The major win (aside from some minor
performance gains), is that this module can now be `npm link`ed into a
babel 6 code base.

.babelrc

@@ -1,3 +1,14 @@
 {
-  "stage": 1
+  "presets": [
+    "stage-1"
+  ],
+  "plugins": [
+    "transform-object-rest-spread",
+    "transform-class-properties",
+    "add-module-exports",
+    "closure-elimination",
+    "transform-decorators-legacy",
+    "transform-es2015-modules-commonjs"
+  ],
+  "sourceMaps": true
 }