kivy/buildozer — pull request #149 by rogererens (2014-10-08 17:06:48+00:00)
Title: Set default ANDROID_SDK_VERSION to 20

On my system (VBox with Ubuntu64 14.04) there is no version/revision 21. Maybe it has been revoked by Google?
There is a revision 20 at this moment.
This might resolve issue https://github.com/kivy/buildozer/issues/146?