Make travis ci spack based

[erikzenker]

...lets wait for travis

[erikzenker]

Okay, there seems to be problems with external repos: Error: Invalid namespace for hzdr.crp repo: builtin

Could be related to: spack/spack#5754

[erikzenker]

Pull request is open, lets wait until it is merged: spack/spack#5787

[erikzenker]

It is not possible to finish a spack build within the travis timeout. @ax3l do you know if there are spack mirrors i could use for that?

[ax3l]

There are no public binary mirrors yet but we could do the following:

• we write a Dockerfile that uses spack [1] [2]
• we publish that Dockerfile, e.g. in the repo itself
• we build the docker image offline and push it to dockerhub (we can also automate that process if it would necessary)
• we do our CI within travis on that docker image (which was partitioned with spack)

It's actually less complicated as it sounds right now :)

Of course we can also just simply set up our own binary mirror, but if people find it we should watch traffic very carefully ;)