Update blinker to 1.4

[pyup-bot]

This PR updates blinker from 1.3 to 1.4.

Changelog

1.4

-----------

Released July 23, 2015

- Verified Python 3.4 support (no changes needed)
- Additional bookkeeping cleanup for non-ANY connections at disconnect
time.
- Added Signal._cleanup_bookeeping() to prune stale bookkeeping on
demand

Links

• PyPI: https://pypi.org/project/blinker
• Changelog: https://pyup.io/changelogs/blinker/
• Homepage: http://pythonhosted.org/blinker/
• Docs: https://pythonhosted.org/blinker/