[DBAL-264] Support for UIDs in PostgreSQL

Add getGuidExpression() to PostgreSqlPlatform so that UUID support
works properly

lib/Doctrine/DBAL/Platforms/PostgreSqlPlatform.php

@@ -666,6 +666,14 @@ class PostgreSqlPlatform extends AbstractPlatform
         return 'TIME(0) WITHOUT TIME ZONE';
     }
 
+    /**
+     * {@inheritDoc}
+     */
+    public function getGuidExpression()
+    {
+        return 'UUID_GENERATE_V4()';
+    }
+
     /**
      * {@inheritDoc}
      */