Minimal set of i386 packages to build on Travis VM

With brute force testing "technique", I end up with only 2 extra i386 packages to install, because:

 * libc6, libgcc1 and gcc-4.6-base are dependencies of libstdc++6 package
 * libstdc++5, libreadline6-dev and libncurses5-dev are not required (at least by this Android project)

**Question in relation to About travis-ci/travis-cookbooks/pull/153**: Do you think we can use the same package list or other Android projects may (often) require more preinstalled libraries by default ???