Unstage yarn.lock pre-commit (facebook#2700)

Since yarn.lock shouldn't be added to the repo, nor excluded via
.gitignore, lets take advantage of lint-staged to remove any staged
yarn.lock files before they can even be committed.

package.json

@@ -29,6 +29,9 @@
     "*.js": [
       "prettier --trailing-comma es5 --single-quote --write",
       "git add"
+    ],
+    "yarn.lock": [
+      "git rm --cached"
     ]
   }
 }