Fix import SUPERUSER_ID in project_issue migration scripts

Found error:

    2014-07-18 17:04:11,710 8772 ERROR v8mig openerp.modules.migration: module project_issue: Unable to load post-migration file project_issue/migrations/8.0.1.0/post-migration.py
    Traceback (most recent call last):
      File "/home/dr/work/openupg/OpenUpgrade/openerp/modules/migration.py", line 167, in migrate_module
        mod = imp.load_module(name, fp, pathname, ('.py', 'r', imp.PY_SOURCE))
      File "/home/dr/work/openupg/OpenUpgrade/addons/project_issue/migrations/8.0.1.0/post-migration.py", line 22, in <module>
        from openerp import SUPERUSER_UID as uid
    ImportError: cannot import name SUPERUSER_UID