Send long Expires headers for static files

SEEK portal runs using the Rails 2 structure. This commit adds changes to
ensure the main static files send an Expires header. These changes are based on
http://bibwild.wordpress.com/2010/09/22/browser-caching-of-rails-assets-for-real/

The existing Rails 3 clause is left in, to ensure caching continues to work
if SEEK is upgraded to Rails 3.