Fast assets:precompile

When static files compile that saves manifest json each time .

My rails project has 1000+ image files.

## before
```
$ time bundle exec rake assets:precompile
bundle exec rake assets:precompile  138.69s user 3.44s system 99% cpu 2:23.01 total
```

## after apply this pull request
```
$ time bundle exec rake assets:precompile
bundle exec rake assets:precompile  5.14s user 1.11s system 92% cpu 6.772 total
```