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)`

Author: posva

build/build.js

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