travis: Pass -O to override

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/.travis.yml b/.travis.yml
index f2fa670..567d295 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -162,4 +162,4 @@
       env:
         - TEST_PY_BD="sandbox"
           BUILDMAN="^sandbox$"
-          OVERRIDE="clang-7"
+          OVERRIDE="-O clang-7"