Merge branch 'nd/travis-gcc-8'

Developer support.  Use newer GCC on one of the builds done at
TravisCI.org to get more warnings and errors diagnosed.

* nd/travis-gcc-8:
  travis-ci: run gcc-8 on linux-gcc jobs

ci/lib-travisci.sh

@@ -99,6 +99,9 @@ export DEFAULT_TEST_TARGET=prove
 export GIT_PROVE_OPTS="--timer --jobs 3 --state=failed,slow,save"
 export GIT_TEST_OPTS="--verbose-log -x"
 export GIT_TEST_CLONE_2GB=YesPlease
+if [ "$jobname" = linux-gcc ]; then
+	export CC=gcc-8
+fi
 
 case "$jobname" in
 linux-clang|linux-gcc)