Fix exception on msf 'db_export' cmd (see rapid7#7008)

Users reported (in GitHub issue rapid7#7008) hitting an exception when attempting to export the contents of the msf database (i.e. workspaces, hosts, events, etc.) via the 'db_export' command.  After some digging, it appears there were a few ActiveRecord changes with the new Rails upgrade that require a couple mods to the way we are querying.

Author: pbarry-r7

lib/msf/core/db_export.rb

@@ -125,15 +125,15 @@ def extract_host_entries
     @owned_hosts = []
     @hosts = myworkspace.hosts
     @hosts.each do |host|
-      if host.notes.find :first, :conditions => { :ntype => 'pro.system.compromise' }
+      if host.notes.where(:ntype => 'pro.system.compromise').first
         @owned_hosts << host
       end
     end
   end
 
   # Extracts all events from a project, storing them in @events
   def extract_event_entries
-    @events = myworkspace.events.find :all, :order => 'created_at ASC'
+    @events = myworkspace.events.order('created_at ASC')
   end
 
   # Extracts all services from a project, storing them in @services