travis: actually use updated compiler versions

For the Linux builds, GCC 8 and 7 and clang 7 were installed, but the
new GCC versions were only used for the coverage build and for building
nghttp2, while the new clang version was not used at all.

BoringSSL needs to use the default GCC as it respects CC, but not CXX,
so it would otherwise pass gcc 8 options to g++ 4.8 and fail.

Also remove GCC 7, it's not needed anymore.

Ref: https://docs.travis-ci.com/user/languages/c/#c11c11-and-beyond-and-toolchain-versioning

Closes https://github.com/curl/curl/pull/3670