MirageOS 2.7.3 发布 unikernels操作系统

MirageOS 2.7.3 发布了。MirageOS是一个构建unikernels的图书操作系统。此更新内容如下：

Improve the Dockerfile (#507, by @avsm)
Use Astring (by @samoht)
Clean-up dependencies automatically added by the tool
- do not require lwt.syntax, cstruct.syntax and sexplib, which should make the default unikernels camlp4-free (#510, #515 by @samoht)
- always require mirage-platform (#512, by @talex5)
- ensure that mirage-types >= 2.6.0 and mirage-types-lwt are installed
Check that the OCaml compiler is at least 4.02.3 (by @samoht)