Added pixbuf cache update to install to properly update loader.cache

loader.cach is being updated to point to the Cellar and not include loaders installed by librsvg. This update fixes that by refreshing the loader cache after install.
`#{HOMEBREW_PREFIX}/lib/gdk-pixbuf-2.0/2.10.0/loaders`; it's not a cellar path.

Also this should be in a post_install method.