Add GCC7 jobs to Travis (#1054)

Add GCC7 jobs to Travis (#1054)

This commit adds two jobs (debug, with portfolio, test groups 0 and 1) to Travis.
Both jobs are added using matrix.include, based on the example in the documentation:
https://docs.travis-ci.com/user/languages/cpp/#GCC-on-Linux . This
unfortunately requires some code duplication but there does not seem to be a
way to do it in a much better fashion.