Only use anzu if Emacs version >= 24

See https://github.com/syohex/emacs-anzu/commit/8e92765ab7ecb821017b4f03fa8678acc4720e53#diff-fea2dd3403e76294136c80b7bee7dbd8
Merge pull request #187 from dcorking/fix-anzu-dependency

Only use anzu if Emacs version >= 24