Add versioning limits to intervaltree module

[cmonr]

Description

PyOCD (required by mbed-host-test) does not do version capping in it's setup.py module requirements, which is pulling a breaking change/latest major release of intervaltree, (3.0.0 was released today). As a result, Travis CI builds are breaking.

This PR caps the module version before checking PyOCD.

Proof that issue is fixed: https://travis-ci.org/cmonr/mbed-os/builds/469097405

Pull request type

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

[rik]

For this kind of second level dependency pinning, I'd recommend using constraints files to keep separate the packages you rely on and the packages they in turn rely on.

[cmonr]

Thanks @rik ! Will definitely be checking that out.

[cmonr]

CI started

[mbed-ci]

Test run: SUCCESS

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