.travis.yml: bump gcc sanitiser job to gcc-9

.travis.yml: bump gcc sanitiser job to gcc-9

The toolchain PPA has it so we might as well use it. We currently have
to add:

  -Wno-error=stringop-truncation

as there are still strncpy operations in the tree operating on things
that haven't been annotated with QEMU_NONSTRING.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>