Pin coverage's version

Coverage 4 appeared on pypi, which is not compatible with old python
interpreters. Pinning coverage to 3.7.1

Signed-off-by: Mate Lakat <mate.lakat@citrix.com>
Reviewed-by: Flavien Quesnel <flavien.quesnel@citrix.com>

GitHub: closes #225 on xapi-project/sm