Avoid unnecessary extra project installation step in Tox
Avoid unnecessary extra project installation step in Tox

- set `skip_install=True` in Tox to avoid re-installing the project
  when it is already installed by dependencies config.
- correct spelling of `skipsdist` directive.

ic-labs/django-icekit,ic-labs/django-icekit,ic-labs/django-icekit,ic-labs/django-icekit