travis: Fix the clang-7 test

When using the OVERRIDE variable we need to pass -O to buildman as well
to use the "override" option to buildman.

Fixed: e9500f49ea ("travis: Use buildman for building with clang")
Signed-off-by: Tom Rini <trini@konsulko.com>

.travis.yml

@@ -389,7 +389,7 @@ matrix:
       env:
         - TEST_PY_BD="sandbox"
           BUILDMAN="^sandbox$"
-          OVERRIDE="clang-7"
+          OVERRIDE="-O clang-7"
     - name: "test/py sandbox_spl"
       env:
         - TEST_PY_BD="sandbox_spl"