Rename: Carton -> Cask
Use Carton in Makefile
Setup Cask in .travis.yml
Make sure .cask/*/elpa is created
Add pye-test-make-environment-with-non-existing-command
Install virtualenv in .travis.yml

It is accidentally removed by
fe0d4a303983ad966ed1ce075126271cefbbf647
Merge branch 'cask'
Properly implement python-environment-run-block
Document python-environment-make-block