Add pyocd depencency to resolve Travis CI issue

[cmonr]

Description

0.15 release of pyOCD cauesd some dependency resolution between it and icetea. icetea unit tests fail when pyOCD is installed due to the invoked icetea command failing to generate JSON.

Reverting pyOCD appears to resolve the issue.

Once 0.15.1 is out (fix has been merged: pyocd/pyOCD#511), this is not specifically needed, but becomes a nice-to-have, and can be removed if desired.

Issues opened related to this PR:

• Update icetea tests to include stderr output of failed command #9390: The test error that the user experiences was completely unintuitive and irritating to root-cause.
• Version pinning in *requirements.txt files icetea#69: icetea does not ues version pinning with their *requirements.txt files
• Travis CI: Bind remaining python modules #9389: Work was already underway to seek out and pin remaining modules, but the update caused one (last? big stir.

NOTE: This is needed for the 5.11.2 RC PR to pass: https://travis-ci.org/ARMmbed/mbed-os/jobs/480215050

Pull request type

[x] Fix
[ ] Refactor
[ ] Target update
[ ] Functionality change
[ ] Docs update
[ ] Test update
[ ] Breaking change

Reviewers

[0xc0170]

CI started

[mbed-ci]

Test run: FAILED

Summary: 1 of 11 test jobs failed
Build number : 1
Build artifacts

Failed test jobs:

• jenkins-ci/mbed-os-ci_greentea-test

[0xc0170]

CI job restarted (was aborted as it got stuck)