small-optimization for mrb_time_zone

Using mrb_str_new instead of mrb_str_new_cstr
small-optimization2 for mrb_time_zone

mrb_str_new_static is more efficient than mrb_str_new in this case.
Merge pull request #1738 from cubicdaiya/issues/optim_mrb_time_zone

small-optimization for mrb_time_zone