Update manifest_version test to pass when gsutil is missing.
Update manifest_version test to pass when gsutil is missing.

This is useful so that users can run unit tests even when they don't
have gsutil installed.

BUG=none
TEST=Run the test without gsutil present. Remote trybot.

Change-Id: I981fb9ceab4c28860d35e8d8880264a1ba4e7fbf
Reviewed-on: https://gerrit.chromium.org/gerrit/44948
Reviewed-by: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@chromium.org>
Commit-Queue: David James <f231dc574bbeb62604660cbc162d1b8642a0a3af@chromium.org>
Tested-by: David James <f231dc574bbeb62604660cbc162d1b8642a0a3af@chromium.org>

chadversary/chromiumos.chromite,chadversary/chromiumos.chromite,bpsinc-native/src_third_party_chromite,bpsinc-native/src_third_party_chromite,bpsinc-native/src_third_party_chromite