Do not create checkpoint_dir relative to current dir

Please backport 19651d2 or remove two unneeded lines from FileNotebookManager.create_checkpoint, which attempt to create checkpoint_dir relative to the current working directory, not the notebooks directory set in NotebookManager.notebook_dir.

For example, if `ipython notebook` is run from `/`, it would attempt to create the directory `/.ipynb_checkpoints` and raise an exception.