Add a few common, compressable types to gzip_types

After switching to Passenger, we found that YSlow and Google PageSpeed were warning about lack of compression on our font and SVG files. This change adds the following types to the `gzip_types`:

 - application/rss+xml
 - application/vnd.ms-fontobject
 - application/x-font-ttf
 - application/xml
 - font/opentype
 - image/svg+xml
 - text/xml