travis: Use -O3 option (GH-5599)

We don't use debugger on Travis..
(cherry picked from commit 8ff5356)

Co-authored-by: INADA Naoki <methane@users.noreply.github.com>

Author: methane

.travis.yml

@@ -8,6 +8,11 @@ cache:
   - pip
   - ccache
 
+env:
+  global:
+    # Use -O3 because we don't use debugger on Travis-CI
+    - CFLAGS="-O3"
+
 branches:
   only:
     - master