Update conditional build to python3

[AndreMiras]

In #1471 we killed crystax for the CI which then means we need to port the conditional build script to python3 see https://github.com/kivy/python-for-android/blob/3e561b0/ci/rebuild_updated_recipes.py#L12

Currently it fails with:

Traceback (most recent call last):
  File "./ci/rebuild_updated_recipes.py", line 99, in <module>
    main()
  File "./ci/rebuild_updated_recipes.py", line 95, in main
    build(target_python, recipes)
  File "./ci/rebuild_updated_recipes.py", line 59, in build
    crystax_ndk_home = os.environ['CRYSTAX_NDK_HOME']
  File "/home/user/venv/lib/python2.7/UserDict.py", line 40, in __getitem__
    raise KeyError(key)
KeyError: 'CRYSTAX_NDK_HOME'

e.g. https://api.travis-ci.org/v3/job/458635355/log.txt