Update ci-requirements.txt
Update ci-requirements.txt

- Remove tox and coveralls
ci-requirements.txt is installed by tox for each tox environment.
That means tox must be installed already.
And coveralls is used outside of tox

- Pin coverage dependency to versions supported by coveralls,
which is installed outside of tox, but must be able to read
reports generated inside of tox.

secure-systems-lab/securesystemslib,secure-systems-lab/securesystemslib