build: use auto for exports

now we don't need to write Vuefire.default in the browser build

BREAKING CHANGE: when using the umd build, do `Vue.use(Vuefire)` instead of
`Vue.use(Vuefire.default)`

build/build.js

@@ -25,7 +25,7 @@ const banner =
 
 const bundleOptions = {
   banner,
-  exports: 'named',
+  exports: 'auto',
   format: 'umd',
   name: moduleName,
 }