Version 2.3.0

Several minor improvements

• jug shell: Add get_filtered_tasks()
• jug: Fix jug --version (which had been broken in the refactoring to use subcommands)
• jug shell: Fix message in jug shell when there are no dependencies (it would repeatedly print the message stating "this will only be run once")
• jug pack: Make it much faster to invalidate elements
• file_store: ensure that the temporary directory exists
• Drops support for Python 3.4