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: NMichal Simek <michal.simek@xilinx.com>
Reviewed-by: NTom Rini <trini@konsulko.com>

.travis.yml

@@ -63,7 +63,7 @@ env:
     - BUILD_DIR=build
     - HOSTCC="cc"
     - HOSTCXX="c++"
-    - QEMU_VERSION="v3.0.0"
+    - QEMU_VERSION="v3.1.0"
 
 before_script:
   # install toolchains based on TOOLCHAIN} variable
@@ -369,6 +369,7 @@ matrix:
         - TEST_PY_BD="vexpress_ca15_tc2"
           TEST_PY_ID="--id qemu"
           QEMU_TARGET="arm-softmmu"
+          QEMU_VERSION="v3.0.0"
           BUILDMAN="^vexpress_ca15_tc2$"
     - name: "test/py vexpress_ca9x4"
       env: