README was outdated since 2.2.7 about FontAwesome integration

Update instruction about FontAwesome integration in README.md
(path has been updated since 2.2.7, import issue).

```css
fontAwesomeEotPath: asset-url("fontawesome-webfont.eot");
fontAwesomeEotPath_iefix: asset-url("fontawesome-webfont.eot?#iefix");
fontAwesomeWoffPath: asset-url("fontawesome-webfont.woff");
fontAwesomeTtfPath: asset-url("fontawesome-webfont.ttf");
fontAwesomeSvgPath: asset-url("fontawesome-webfont.svg#fontawesomeregular");
import "fontawesome/font-awesome";
```

https://github.com/seyhunak/twitter-bootstrap-rails/issues/606 : FontAwesome probably didn't load at all
https://github.com/seyhunak/twitter-bootstrap-rails/issues/593 : Based on this issue, thank you @ano1chan and @victorbstan