Update to 2.04

Upstream changes:
2.04 2016-05-07T15:38:37Z
    - Quiet compile time warnings about function prototypes and vars being
      used only once

2.03 2016-05-06T22:27:44Z
- Rewording some documentation
- No longer require an explicit version of perl in META.json or cpanfile

2.02 2016-05-06T21:56:10Z
- Create mutable clones of readonly structures with Readonly::Clone
- Still not convinced this is useful but... fixes #13
- Minor typo patch from Gregor Herrmann <gregoa@debian.org> fixes #21

(wen)