Fix mod_tags_similar in postgres

## Executive Summary
The similar tags module throws an uncaught SQL exception in postgres breaking any page it's loaded on

## Testing Instructions
Install the testing dataset on postgres. The main home page will throw an SQL error. Apply the patch. This error will disappear and the page will render normally.

## Things to note
There's an issue with the fact the same page is shown 3 times still in the module - but until I can work out errors  with saving articles with tags treat this as a known issue. The fact that mysql with the same articles and associated tags shows 0 articles is just as concerning :P So let's just allow people to navigate on their site first!