v1.2.3: Upgrade pyas2lib and pytest-django to the latest versions (#96)

• Bump version of pyas2lib to 1.4.3
• Update variables in run_post_receive to fit the meaning (#82 by @timfanda35)
• Fix link to AUTHORS since now it's a Markdown file (#85 by @adiroiban)
• Update the lengths of the payload fields to allow longer file names (#87 by @pouldenton)
• Update documentation to use django-admin instead of django-admin.py (#89 by @bkc)