Install composer on Appveyor
Appveyor doesnt include PHP, so that needs to be installed, and then composer on top.
this help page contains an example install using Chocolatey (see also #30), but also encountered problems.
9e8c6a57 was the manual composer installation on Linux by @damngamerz , which was removed in 317cde5e..5ae34b5b due to it being available on Debian sid.