Fixed _skip_files pattern on GAE 1.9.1

as an alternative, we can add ```--allow_skipped_files``` option
in run.py
``` python
run_command = '''
      dev_appserver.py %s
      --host %s
      --port %s
      --admin_port %s
      --storage_path=%s
      --clear_datastore=%s
      --skip_sdk_update_check
      --allow_skipped_files
    ''' % (DIR_MAIN, ARGS.host, port, port + 1, DIR_STORAGE, clear)
```

and simplify appengine_config.py
``` python
import os
import sys

if os.environ.get('SERVER_SOFTWARE', '').startswith('Google App Engine'):
  sys.path.insert(0, 'lib.zip')
else:
  sys.path.insert(0, 'lib')
```

but it will cause big difference between dev/production environment (dev server will have access to all ```skip_files```)