README.rst: fix badge url (GH-137)
README.rst: fix badge url (GH-137)

- Change to https url
- Fix target url (prev url was 404)
- remove `latest` version
jeff5/jython-devguide,jeff5/jython-devguide,ezio-melotti/devguide,jython/devguide,jython/devguide,jeff5/jython-devguide,jython/devguide