[CI] Fix test running with python 2

[danimtb]

In the development version of conan (1.22), python 2 is deprecated on purpose.

We should set USE_UNSUPPORTED_CONAN_WITH_PYTHON_2 variable in the CI for Conan dev > 1.21.X

Related to #145

[jgsogo]

Add it to the .travis.yml and appveyor.yml files in the entries for current conandev (we will need to move it when rolling releases, but eventually it will disappear), IMO it is better than adding it to the actual testing sources