Travis updates.

* sg/travis-linux32-sanity:
  travis-ci: don't fail if user already exists on 32 bit Linux build job
  travis-ci: don't run the test suite as root in the 32 bit Linux build
  travis-ci: don't repeat the path of the cache directory
  travis-ci: use 'set -e' in the 32 bit Linux build job
  travis-ci: use 'set -x' for the commands under 'su' in the 32 bit Linux build