travis: Setup QEMU_VERSION as variable

This change enables setting up specific Qemu version or sha1 for new
targets which are added after (current) v3.0.0 version.
This changes is preparation step for adding new Xilinx Versal Virt
platform which was merge after v3.0.0.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Tom Rini <trini@konsulko.com>