update to standard v8

The only thing we have to change is to self-close tags that don't
contain anything. This wasn't even an explicit change in standard. It
was already a rule, but I think it wasn't getting enforced very well
until a bugfix.

package.json

@@ -60,7 +60,7 @@
     "plist": "^1.2.0",
     "rimraf": "^2.5.2",
     "run-series": "^1.1.4",
-    "standard": "^7.0.0"
+    "standard": "^8.0.0-beta.5"
   },
   "engines": {
     "node": ">=4.0.0"