Update Readme ... replace CI badge

[cehbrecht]

Description

We are using know GitHub actions ... but badge is still pointing to Travis.

Environment

• operating system:
• Python version:
• PyWPS version: 4.4.1
• source/distribution
  • [ ] git clone
  • [ ] Debian
  • [ ] PyPI
  • [ ] zip/tar.gz
  • [ ] other (please specify):
• web server
  • [ ] Apache/mod_wsgi
  • [ ] CGI
  • [ ] other (please specify):

Steps to Reproduce

Additional Information

[cehbrecht]

fixed by PR #592.