update default.rb to include android build tools

Explicitly add the build-tools rev 20 to the components group. Ant fails when there are no build tools, and somewhere the build-tools-* package didn't get installed. r20 is the latest as of 2014-8-18, but will need to be updated.

Basic proof: https://github.com/indrora/Atomic/compare/359a055aaf53...8096f5774697

Failing build: https://travis-ci.org/indrora/Atomic/builds/32836533

Passing build: https://travis-ci.org/indrora/Atomic/builds/32880563