Set compiler requirement/preference correctly for spack 1.0

[msimberg]

Currently, spack will warn about the following when building a stack:

Warning: The packages:all:compiler preference has been deprecated in Spack v1.0, and is currently ignored. It will be removed from config in Spack v1.2.

We should update the packages.yaml config to use packages:all:require:['%$compiler'] or packages:cxx:require:$compiler.

[msimberg]

#248 removed the warning by simply removing the preference altogether. This still needs to be set properly (like in #251).

[bcumming]

Fixed in #251