Support ABT v.20 and deprecate anything less than v.17
Support ABT v.20 and deprecate anything less than v.17

Fixes #97 and #100. Shout-out to @worrel for writing the original pull request.

kenrestivo/lein-droid,clojure-android/lein-droid,dferens/lein-droid,celeritas9/lein-droid,kenrestivo/lein-droid,celeritas9/lein-droid,clojure-android/lein-droid,nablaa/lein-droid