Travis: install packages with more narrow versions

[0xc0170]

Description

We had an issue yesterday as one package was updated to new minor version and broke our tools tests. This is the current line in travis config file: pip install pytest pylint hypothesis==3.88.3 mock coverage coveralls for the job tools-pytest

We should be more strict with versions (like hypothesis, that one could be updated to use any <4.0 for instance) to avoid breakages like we experienced this week.

cc @ARMmbed/mbed-os-tools

Issue request type

[ ] Question
[X] Enhancement
[ ] Bug

[ciarmcom]

Internal Jira reference: https://jira.arm.com/browse/MBOCUSTRIA-453

[cmonr]

Fixing with #9389