Fix Python 2.6 on Appveyor CI

[shakeelmohamed]

Add version ceiling on cryptography transitive dependency

Fixes #52

Somewhere in the dependency tree, cryptography is being installed.
In the recent 2.1 release of the cryptography package, Python 2.6
was deprecated.

[coveralls]

Coverage remained the same at 81.692% when pulling 7a71518 on shakeelmohamed:ci/fix-appveyor-python2.6 into 8c52a67 on mayn:master.

[mayn]

@shakeelmohamed thanks for your PR!
i saw that also... lets see what appveyor thinks.
nice write-up.

[shakeelmohamed]

@mayn Now Python 3.3 fails to build the cryptography package! 😄 Is there a way to conditionally set this restriction for Python 2.6 only?

[mayn]

@shakeelmohamed didn't see that one coming. 😄
Assumingly you should be able to implement an conditional pin for python 2.6 appveyor environment for the tox profile (https://github.com/mayn/packerlicious/blob/master/tox.ini#L21)

Might require minor refactor to the tox profile for appveyor also (not 100% sure)

see:
http://tox.readthedocs.io/en/latest/config.html#generating-environments-conditional-settings

[coveralls]

Coverage remained the same at 81.692% when pulling 12b11f2 on shakeelmohamed:ci/fix-appveyor-python2.6 into 8c52a67 on mayn:master.

[coveralls]

Coverage remained the same at 81.692% when pulling 256ae95 on shakeelmohamed:ci/fix-appveyor-python2.6 into 8c52a67 on mayn:master.

[shakeelmohamed]

This solution works, but it's a bit clunky since it bring in another requirements file. Changing the tox config didn't seem to work, I'm guessing that's because pip install -r requirements-travis.txt was happening before tox in both CI scenarios. (So, it didn't matter which version of cryptography was specified in the tox file since it was already failing to install v2.1 before that point)

One possible solution would be moving the CI installations to tox.ini, but that could add some bloat for local devs - a worthwhile tradeoff perhaps.

[mayn]

hey this is great.
Given that this is only for the build environments and tox -e py26 --recreate still works locally for users, I can live with the clunk.