PR Description

Travis: Fix old default osx_image with xcode12.2 and remove non used boost cmake references.

Checklist

[x] Doxygen documentation of the code completed (classes, methods, types, members...)
[x] Check if it follows the tools structure described in CONTRIBUTING.md
[x] New entry in the ChangeLog.md added.
[x] Update the readme with potentially a screenshot of the tools if it applies.
[x] No warning raised in Debug cmake mode (otherwise, Travis C.I. will fail).