Use the updated is_dirty API

The pull request gitpython-developers/GitPython#294 has been merged but the API
was changed slighty. This commit reflects this change so that it works again.

PyGitUp/git_wrapper.py

@@ -134,7 +134,7 @@ def stash(self):
         """
         stashed = False
 
-        if self.repo.is_dirty(consider_submodules=False):
+        if self.repo.is_dirty(submodules=False):
             if self.change_count > 1:
                 message = 'stashing {0} changes'
             else: