travis: Switch QEMU to 3.1.0 version

Vexpress ca15_tc2 is failing with 3.1.0 because of QEMU issue.
When this patch is applied
https://patchwork.kernel.org/patch/10754401/


Vexpress can be also turn to newer QEMU version.

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