Fix bug, TypeError: unicode argument expected, got 'str' on core management validation

Error:

	2013-04-03 02:37:08 [71497] [INFO] Starting gunicorn 0.17.2
	2013-04-03 02:37:08 [71497] [INFO] Listening at: http://127.0.0.1:8000 (71497)
	2013-04-03 02:37:08 [71497] [INFO] Using worker: sync
	2013-04-03 02:37:08 [71500] [INFO] Booting worker with pid: 71500
	2013-04-03 02:37:10 [71500] [ERROR] Exception in worker process:
	Traceback (most recent call last):
	  File "/Users/avelino/.virtualenvs/yacows.virgula/lib/python2.7/site-packages/gunicorn/arbiter.py", line 485, in spawn_worker
		worker.init_process()
	  File "/Users/avelino/.virtualenvs/yacows.virgula/lib/python2.7/site-packages/gunicorn/workers/base.py", line 100, in init_process
		self.wsgi = self.app.wsgi()
	  File "/Users/avelino/.virtualenvs/yacows.virgula/lib/python2.7/site-packages/gunicorn/app/base.py", line 103, in wsgi
		self.callable = self.load()
	  File "/Users/avelino/.virtualenvs/yacows.virgula/lib/python2.7/site-packages/gunicorn/app/djangoapp.py", line 101, in load
		return mod.make_wsgi_application()
	  File "/Users/avelino/.virtualenvs/yacows.virgula/lib/python2.7/site-packages/gunicorn/app/django_wsgi.py", line 36, in make_wsgi_application
		if get_validation_errors(s):
	  File "/Users/avelino/.virtualenvs/yacows.virgula/lib/python2.7/site-packages/django/core/management/validation.py", line 150, in get_validation_errors
		e.add(opts, "'%s' has a relation with model %s, which has either not been installed or is abstract." % (f.name, f.rel.to))
	  File "/Users/avelino/.virtualenvs/yacows.virgula/lib/python2.7/site-packages/django/core/management/validation.py", line 19, in add
		self.outfile.write(self.style.ERROR(force_str(u"%s: %s\n" % (context, error))))
	TypeError: unicode argument expected, got 'str'
	Traceback (most recent call last):
	  File "/Users/avelino/.virtualenvs/yacows.virgula/lib/python2.7/site-packages/gunicorn/arbiter.py", line 485, in spawn_worker
		worker.init_process()
	  File "/Users/avelino/.virtualenvs/yacows.virgula/lib/python2.7/site-packages/gunicorn/workers/base.py", line 100, in init_process
		self.wsgi = self.app.wsgi()
	  File "/Users/avelino/.virtualenvs/yacows.virgula/lib/python2.7/site-packages/gunicorn/app/base.py", line 103, in wsgi
		self.callable = self.load()
	  File "/Users/avelino/.virtualenvs/yacows.virgula/lib/python2.7/site-packages/gunicorn/app/djangoapp.py", line 101, in load
		return mod.make_wsgi_application()
	  File "/Users/avelino/.virtualenvs/yacows.virgula/lib/python2.7/site-packages/gunicorn/app/django_wsgi.py", line 36, in make_wsgi_application
		if get_validation_errors(s):
	  File "/Users/avelino/.virtualenvs/yacows.virgula/lib/python2.7/site-packages/django/core/management/validation.py", line 150, in get_validation_errors
		e.add(opts, "'%s' has a relation with model %s, which has either not been installed or is abstract." % (f.name, f.rel.to))
	  File "/Users/avelino/.virtualenvs/yacows.virgula/lib/python2.7/site-packages/django/core/management/validation.py", line 19, in add
		self.outfile.write(self.style.ERROR(force_str(u"%s: %s\n" % (context, error))))
	TypeError: unicode argument expected, got 'str'
	2013-04-03 02:37:10 [71500] [INFO] Worker exiting (pid: 71500)
	2013-04-03 02:37:10 [71497] [INFO] Shutting down: Master
	2013-04-03 02:37:10 [71497] [INFO] Reason: Worker failed to boot.