buildman: Add in gcc-8.1.0 toolchains

There are now (and have been for a while) gcc-8.1.0 toolchains available
on kernel.org in the style we have been using already.  Add that as the
first choice for toolchains now and update .travis.yml to reflect this.

Signed-off-by: Tom Rini <trini@konsulko.com>
3 files changed