Automatic build by xtensor xstack

[constantinpape]

QuantStack have an automated build system that tests several
xtensor dependencies against the latest xtensor master.
See http://195.201.91.244:8080/teams/main/pipelines/xstack.
It would be great to include z5 in there, to know about breaking changes like #111 as early as possible.

The build recipes are located here
https://github.com/xtensorgistbucket/droneme/.

On first glance this looks pretty straightforward, we probably just have to combine the travis set-up of z5 and one of the build scripts from xstack.

cc @wolfv