Skip persistence test for MultiContext - Py2

[rahulporuri]

The underlying traits pickling machinery seems to be broken with the latest release i.e. 5.0.0, leading to broken pickling of MultiContext instances on Python 2.

Given the move to Python 3, this has been deemed an acceptable bug on Python 2 so we skip the failing test.

Once this PR is merged, PR #25 and #27 can be updated to get CI working and the release process for the new release of codetools can be started.

[rahulporuri]

Sigh. The CI is failing now because of the traits.protocols.api import, which got fixed in #27 . Instead of merging this PR into master, I'm going to update it to get merged into PR #27

[rahulporuri]

Sigh. All of the push builds fail with the same error

======================================================================
ERROR: Failure: ImportError (No module named protocols.api)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/travis/virtualenv/python2.7.14/lib/python2.7/site-packages/nose/loader.py", line 418, in loadTestsFromName
    addr.filename, addr.module)
  File "/home/travis/virtualenv/python2.7.14/lib/python2.7/site-packages/nose/importer.py", line 47, in importFromPath
    return self.importFromDir(dir_path, fqname)
  File "/home/travis/virtualenv/python2.7.14/lib/python2.7/site-packages/nose/importer.py", line 94, in importFromDir
    mod = load_module(part_fqname, fh, filename, desc)
  File "/home/travis/build/enthought/codetools/codetools/contexts/tests/with_mask_test_case.py", line 12, in <module>
    from codetools.contexts.with_mask import Mask
  File "/home/travis/build/enthought/codetools/codetools/contexts/with_mask.py", line 124, in <module>
    from traits.protocols.api import adapt
ImportError: No module named protocols.api
----------------------------------------------------------------------

which can't be fixed, like the PR builds.

[dpinte]

@rahulporuri the PR testing is green. We should not have the second push test. I think this ok to merge