bump version to 0.4.0
bump version to 0.4.0

Changes since 0.3.0:
- The magic constants NULL/CAPTURE/STDOUT/STDERR have been replaced with
  dedicated methods: null_*/capture_*/stdout_to_stderr/stderr_to_stdout.
  This saves you the trouble of extra imports, and it avoids weird
  typechecking issues like .stdin(CAPTURE) (if that's supposed to be a
  type error, then would need need two different types for NULL).
- The env_remove and env_clear methods have been dropped, and we've gone
  back to the env/full_env API. (Though this time around they're methods
  instead of flags.) This is easier to use and more flexible, and it
  also prevents weird race conditions.
- The cwd method has been renamed to dir. The cmd and cwd names were too
  awkwardly close to each other.

oconnor663/duct.py,oconnor663/duct