.travis.yml: rationalise clang testing

.travis.yml: rationalise clang testing

As Travis includes Clang 5.0 in its own build environment there is no
point manually building with older Clangs. We still need to test with
the two pythons though so we leave them as minimal system only builds.
We also split the clang build into two as it often exceeds the 40
minute build time limit.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Tested-by: Philippe Mathieu-Daudé <f4bug@amsat.org>