Add python:i386 to gcc 32-bit builds to avoid a strange dependency
issue with the gcc-4.9 and gcc-7 builds.
Remove chrome workaround since it's not needed anymore.
Change 32-bit clang dependency to gcc-7-multilib (no real reason, I just
feel like doing so).
[skip appveyor]
Also make the build noisier to prevent some build failures when download
speeds are slow from whatever repository.
Note: The 64-bit devel build currently doesn't benefit from ccache for
whatever reason.
[skip appveyor]
Build with gcc-5 (whatever the ubuntu toolchain ppa uses - currently
gcc-5.2), gcc-4.9 and clang-3.7.
Instead of adding everything into the yml file, I've split off most of
the work into a separate shell script. It's easier to maintain and
extend that way, and Travis CI also recommends to do it this way.
Jonathan Li