Minimal set of i386 packages to build on Travis VM

The list of i386 packages to install can be reduced, because:

 * libc6:i386, libgcc1:i386 and gcc-4.6-base are dependencies of libstdc++6:i386 package
 * libstdc++5:i386, libreadline6-dev:i386 and libncurses5-dev:i386 are not required (by this project)
Merge pull request #4 from gildegoma/travis-i386-deps

Minimal set of i386 packages to build on Travis VM