Use ci-requirements in tox.ini
Use ci-requirements in tox.ini

Travis is set up to use tox to install and initiate testing.  Set tox.ini to use ci-requirements.txt instead of requirements.txt.  The latter can be used by users to install dependencies that are only needed to run securesystemslib.

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