Add App::cpanminus

This uses the cpanm script from the github repo (avoiding the xrl.us and
cpanmin.us aliases) to install the App::cpanminus dist from CPAN.
Merge pull request #2 from zakame/feature/add-cpanm

Add App::cpanminus
Added Perl 5.20.1
Remove 5.20.0 (replaced by 5.20.1)
Update README.md

Add a link to the docker repo, as suggested in https://github.com/Perl/docker-perl/issues/3
5.18.2 -> 5.18.4, fix for gcc4.9, and sha1 check
Added SHA-1 checking for 5.20.1, unthreaded
Added SHA-1 checking for 5.20.1, threaded
Use fewer steps, but othewise no change
Just whitespace (auto-regenerated the files)
Added a helper script to regen the Dockerfiles