Publish version tag

Add 'latest' tag manually for now (doing both would create two separate
commits, while publish-to-git does not support adding multiple tags;
see <https://github.com/Rush/publish-to-git/issues/6>)

package.json

@@ -13,7 +13,7 @@
   "main": "lib",
   "scripts": {
     "build": "babel -d lib src",
-    "postpublish": "publish-to-git --force --tag latest",
+    "postpublish": "publish-to-git --force",
     "prepublish": "npm run build",
     "test": "ava"
   },