Do not include unuseful comments when precompiling assets

The head of the CSS file uses "/*!". It prevents Sprockets to delete the comments for the final asset CSS files. All those comments are outputed to the final CSS file like this:
```
...(blah blah blah)/*
 *  Font Awesome 3.2.1
 *  the iconic font designed for Bootstrap
 *  ------------------------------------------------------------------------------
 *  The full suite of pictographic icons, examples, and documentation can be
 *  found at http://fontawesome.io.  Stay up to date on Twitter at
 *  http://twitter.com/fontawesome.
 *
 *  License
 *  ------------------------------------------------------------------------------
 *  - The Font Awesome font is licensed under SIL OFL 1.1 -
 *    http://scripts.sil.org/OFL
 *  - Font Awesome CSS, LESS, and SASS files are licensed under MIT License -
 *    http://opensource.org/licenses/mit-license.html
 *  - Font Awesome documentation licensed under CC BY 3.0 -
 *    http://creativecommons.org/licenses/by/3.0/
 *  - Attribution is no longer required in Font Awesome 3.0, but much appreciated:
 *    "Font Awesome by Dave Gandy - http://fontawesome.io"
 *
 *  Author - Dave Gandy
 *  ------------------------------------------------------------------------------
 *  Email: dave@fontawesome.io
 *  Twitter: http://twitter.com/byscuits
 *  Work: Lead Product Designer @ Kyruus - http://kyruus.com
 */...(blah blah blah)
```

It's a bandwidth waste for production environment. Let's remove it.