Enable limit-usize-gap in CI tests.

Considering the new usize calculation will be default soon, add the
config option in for Travis, Cirrus and appveyor.

scripts/gen_travis.py

@@ -191,6 +191,7 @@ def format_job(os, arch, combination):
     if len(malloc_conf) > 0:
         configure_flags.append('--with-malloc-conf=' + ','.join(malloc_conf))
 
+    configure_flags.append('--enable-limit-usize-gap')
     if not compilers:
         compiler = GCC.value
     else: