feature/#487_Use_only_the_newest_compilers_for_standard_pipeline into master

closes #487

From now on standard pipelines compile phx with clang, gcc7.3.1 and msvc. Nightlies are compiled as before (+gcc5.3.1, +gcc6.3.1).

merged

By Jonathan Ehret on 2018-07-16T07:49:49 (imported from GitLab)

mentioned in commit 2dcbc67f

By Jonathan Ehret on 2018-07-16T07:49:50 (imported from GitLab)