Update status images to match the Travis CI style

I used this script to generate them:
https://github.com/mgedmin/shields/blob/python-badge-server/badgeserver.py
(instructions at
https://github.com/olivierlacan/shields/issues/24#issuecomment-16221676)

  ./badgeserver.py
  wget -O status_image_success.png "http://localhost:8000/image.png?vendor=winbot&status=passing&color=green"
  wget -O status_image_failure.png "http://localhost:8000/image.png?vendor=winbot&status=failing&color=red"
  wget -O status_image_exception.png "http://localhost:8000/image.png?vendor=winbot&status=error&color=red"
  wget -O status_image_warnings.png "http://localhost:8000/image.png?vendor=winbot&status=warnings&color=yellow"
  wget -O status_image_skipped.png "http://localhost:8000/image.png?vendor=winbot&status=skipped&color=lightgray"

Fixes #1