Make plugin compatible with postgresql

lib/last_updated_by_issue_patch.rb uses backquotes around some of the database column names in the sql query it builds. This is illegal in postgresql and causes errors such as:


    ActionView::Template::Error (PG::Error: ERROR:  syntax error at or near "`"
    LINE 1: ...journals as j1 inner join users as u on u.id = j1.`user_id` ...
                                                             ^
    : select CONCAT_WS(' ', u.firstname, u.lastname) as name, u.id as id from journals as j1 inner join users as u on u.id = j1.`user_id` where `journalized_type` = 'Issue' and `journalized_id` = 4354 order by j1.`id` DESC limit 1):

This patch removes these backquotes, which should be compatible with mysql and sqlite as well.