Fix for [DDC-2506] CTI JOINs and WITH conditionals

http://www.doctrine-project.org/jira/browse/DDC-2506

Moved the `SqlWalker::walkConditionalExpression()` out of `walkJoin()` and into `walkJoinAssociationDeclaration()` after the base class JOIN but **before** the `_generateClassTableInheritanceJoins()`, thus allowing WITH conditions in Class Table Inheritance joins to apply correctly.

On a side note, I'm not able to get phpunit to run (even on a clean Master) so I'm leaving the testing up to Travis.