.travis.yml: enable 32-bit builds

As Ubuntu supports running 32-bit executables based on its multiarch
setup, we can enable the tests on 32-bit configurations as well.

Currently, valgrind fails for 32-bit executables on the 64-bit host.
This requires some more investigation.
.travis.yml: install libc6-dbg:i386 to run valgrind on 32-bit
.travis.yml: add g++-multilib/libc6-dbg:i386 only on BITS=32
Merge pull request #103 from pah/travis/32bit

.travis.yml: enable 32-bit builds