pg_search plugin: check if pg_search gem installed

the pg_search gem was installed "by hand" but I couldn't enable the pg_search plugin:

sh stdout: W: failed to install pg_search; not enabling
sh stderr: dpkg-query: package 'ruby-pg-search' is not installed and no information is available
Use dpkg --info (= dpkg-deb --info) to examine archive files,
and dpkg --contents (= dpkg-deb --contents) to list their contents.
E: Unable to locate package ruby-pg-search