travis: add 32 bit build, minor updates and cleanups

travis: add 32 bit build, minor updates and cleanups

- Rejig the Travis file so it does a 32 bit Linux build (albeit on a 64
  bit system). GlusterFS is only installed on 64 bit Linux because the
  32 bit deb tries to pull in psmisc:i386 which conflicts with the
  regular (64 bit) psmisc. The clang 32 bit build works fine but only
  the gcc 32 bit build is done to reduce the number of builds for now.
- Minor reformatting to make it a bit easier to read the shell script
  (but behind the scenes it's still all squashed on to one line).
- Stop apt from installing/upgrading non-essential packages so as to
  reduce build time.
- Make OSX build use a newer Xcode.