Revert "update default.rb to include android build tools"

Sorry @indrora @BanzaiMan, but I disagree about travis-ci/travis-cookbooks#359 and I propose to revert it. 

The `build-tools-x.y.z` components are intentionally not preinstalled in the VM image (see travis-ci/travis-ci#2296 for more details), to force each project to explicitly declare their required version (exactly like @indrora did in https://github.com/indrora/Atomic/commit/8096f5774697e6562328a071502c2776459d1fe9).

See also our recommendation about http://docs.travis-ci.com/user/languages/android/#Pre-installed-components.