Update installation to consider symlinks as well.

Addresses problem raised in #25

install_and_cache_pkgs.sh

@@ -59,7 +59,7 @@ for package in ${normalized_packages}; do
       # Pipe all package files (no folders) to Tar.
       dpkg -L "${cache_package_name}" |
         while IFS= read -r f; do     
-          if test -f $f; then echo "${f:1}"; fi;  #${f:1} removes the leading slash that Tar disallows
+          if test -f $f || test -L $f; then echo "${f:1}"; fi;  #${f:1} removes the leading slash that Tar disallows
         done | 
         xargs tar -czf "${cache_filepath}" -C /      
       echo "done (compressed size $(du -k "${cache_filepath}" | cut -f1))."