asset_path added to sprites.css.erb

Tried to switch to (or add) glyphicons in bootstrap_and_overrides.css but the path to glyphicons was wrong
changed it so it used asset_path 

Without this when you modify bootstrap_and_overrides.css to use glyphicons the path to the fonts is wrong
/*

    Use Font Awesome icons (default)
    To use Glyphicons sprites instead of Font Awesome, replace with "require twitter-bootstrap-static/sprites
    =require twitter-bootstrap-static/fontawesome
*/