gitlab-ci: Only build x86_64-deb8 and fedora27 for releases
gitlab-ci: Only build x86_64-deb8 and fedora27 for releases

These are largely redundant as they are covered by x86_64-deb9.

sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc