gcc 7.x should be compiled with gcc 7.x

[mikrosk]

It's not so hard: https://docs.travis-ci.com/user/languages/cpp/#C11-C%2B%2B11-(and-Beyond)-and-Toolchain-Versioning

Just to be sure that the other projects (mintlib, pml) are still compilable. For instance, gcc 5.x has shifted from default C89 standard and one had to introduce -fgnu89-inline to mintlib as it's full of incompatible inline statements.

[miniupnp]

well, we could just "expand the matrix" and compile with both gcc-7 and the default gcc.
That would be at travis-ci cost :)