change plugin spec requirement from keg to flask

- nothing keg-specific here for dependencies
fixes #30

Author: guruofgentoo

keg_storage/__init__.py

@@ -1,7 +1,7 @@
 from keg_storage.version import VERSION as __version__  # noqa
 from importlib.util import find_spec as _find_spec
 
-if _find_spec('keg'):
+if _find_spec('flask'):
     from keg_storage.plugin import (  # noqa: F401
         Storage,
         LinkViewMixin,