Remove debian8-gcc from gitlab-ci
The debian8 image is not compatible with current dune version. It is restricted to
- gcc < 5.0
- cmake < 3.1
This results in failing piplines, since some dependent modules can not be configured with cmake < 3.1. Also, gcc 4.9.2. is probably not fully c++14 compatible.
So, I suggest, to remove this gitlab-ci job from .gitlab-ci.yml