Update Lib/CakeMigration.php

This is to fix errors when I run `Console/cake Migrations.migration run all -p Migrations
`

The errors are:
Cake Migration Shell
---------------------------------------------------------------
Strict Error: Only variables should be assigned by reference in [APP/Plugin/Migrations/Lib/CakeMigration.php, line 510]




Strict Error: Only variables should be assigned by reference in [APP/Plugin/Migrations/Lib/CakeMigration.php, line 510]




Strict Error: Only variables should be assigned by reference in [APP/Plugin/Migrations/Lib/CakeMigration.php, line 510]




Running migrations:
All migrations have completed.

The Strict Error has to do with PHP 5.4

From php's manual on assignment by reference: http://php.net/manual/en/language.operators.assignment.php

As of PHP 5, the new operator returns a reference automatically, so assigning the result of new by reference results in an E_DEPRECATED message in PHP 5.3 and later, and an E_STRICT message in earlier versions.