Default to Python 3.6 instead of 3.7

[tkf]

fixes #127

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Checking with PyCall.jl:

• https://github.com/JuliaPy/PyCall.jl/pull/590
  • https://travis-ci.org/JuliaPy/PyCall.jl/builds/440930322
  • https://ci.appveyor.com/project/StevenGJohnson/pycall-jl-nu3aa/builds/19480780

[stevengj]

If we do this, what is the upgrade path for existing users when we eventually switch to 3.7? We don't want to tell everyone to Conda.free("python=3.6") manually…

[stevengj]

One option would be to use MINICONDA_VERSION="3.6", since this string is now saved between updates … if we notice that this number has increased we could free the old version.

[tkf]

The easiest (non?) solution is to just downgrade Python to 3.6 without pinning. It's then almost equivalent to the current situation before this PR except that it reduces the probability of accidental downgrade. Python would be automatically upgraded by Conda.add(package) once 3.6 becomes too old to install the package.

If you want a more controlled solution then I guess we can check Python version in build.jl or __init__? Automatic upgrade in build.jl is probably rather too brutal but maybe we can print something like "Your Python version is becoming old. Run Codna.update() to upgrade Python and Python packages."

[tkf]

I think https://github.com/JuliaPy/PyCall.jl/pull/613 is a better solution. It may still make sense to add Conda.pin and Conda.free.