Remove assignment by reference

Using the =& assignment by reference was deprecated in php 5.3+ .

On my setup (Ubuntu 13.04 & PHP 5.4.9-4ubuntu2 (cli)) cake shell throws a misleading error at the changed line:

"Fatal Error Error: Class 'ConsoleOutput' not found in [/vagrant/site_deploy/Plugin/Migrations/Console/Command/MigrationShell.php, line 86]"


I noticed this has been fixed in the develop branch. Just thought it might be helpful to others to add this quick fix before develop branch is ready.