remove support of python 2.6 (#4325)

Jira: RHINENG-12167

- this change should not be applied to the "rhel_6" branch
- only explicit support of py26 is removed
- remove unused files/modules, e.g.
  .collections.py, Jenkinsfile, shippable.yml
  insights/contrib/ElementPath/Tree.py
  insights/contrib/importlib.py

Signed-off-by: Xiangce Liu <[email protected]>

Author: xiangce