Don't normalise assets by default

I think we need to return `normalize_asset_timestamps` option from Capistrano 2.x (https://github.com/capistrano/capistrano/blob/1fd63128120115657acec102fec573ca9d9e88e8/lib/capistrano/recipes/deploy.rb#L293), but it should be __disabled__ by default (this operation is not necessary at all for Rails >= 3.1).

Otherwise (as `public/javascripts`, `public/stylesheets`, `public/images` doesn't exist in basic Rails >= 3.1 app) it generates some ugly warnings:

```
 INFO [d39e3799] Running find public/images public/javascripts public/stylesheets -exec touch -t 201308052135.27 {} ';'; true on 37.139.8.xxx
DEBUG [d39e3799] Command: cd /home/deploy/navigator/releases/20130805213527 && find public/images public/javascripts public/stylesheets -exec touch -t 201308052135.27 {} ';'; true
DEBUG [d39e3799]  find:
DEBUG [d39e3799]  `public/images'
DEBUG [d39e3799]  : No such file or directory
DEBUG [d39e3799]
DEBUG [d39e3799]  find:
DEBUG [d39e3799]  `public/javascripts'
DEBUG [d39e3799]  : No such file or directory
DEBUG [d39e3799]
DEBUG [d39e3799]  find:
DEBUG [d39e3799]  `public/stylesheets'
DEBUG [d39e3799]  : No such file or directory
DEBUG [d39e3799]
 INFO [d39e3799] Finished in 0.143 seconds command successful.
```