Update user_agents.php

Added Win 8.1 user agent to the array
Added post-increment for $count

It has been added the missing post-increment for variable $count.
Merge pull request #2768 from porquero/patch-1

Added post-increment for $count
Merge pull request #2753 from SirDre/patch-1

Update user_agents.php
[ci skip] Some spaces & docblock fixes