.shippable.yml: HACK: install latest west

After some discussion, we've decided to ensure that shippable runs
with the latest west bootstrapper on PyPI by making that happen in
.shippable.yml for now, rather than updating our CI container.

Signed-off-by: Marti Bolivar <marti@foundries.io>

.shippable.yml

@@ -27,6 +27,7 @@ build:
 
     ci:
       - export CCACHE_DIR=${SHIPPABLE_BUILD_DIR}/ccache/.ccache
+      - pip3 install -U west
       - >
         if [ "$IS_PULL_REQUEST" = "true" ]; then
           ./scripts/ci/run_ci.sh -c -b ${PULL_REQUEST_BASE_BRANCH} -r origin -m ${MATRIX_BUILD} -M ${MATRIX_BUILDS} -p ${PULL_REQUEST};