travis: streamline config

travis: streamline config

- Reorder the testing matrix to more or less the order of importance.
- Don't install libc and libstdc++ explicitly; it's not necessary and
  for some reason it installs a bunch of other packages.
- We only test on Linux, so get rid of the conditional.
- Don't error out if apt update fails, since that seems to happen every
  now and then because some repository changed keys.

Signed-off-by: Omar Sandoval <osandov@fb.com>