[PATCH] travis-ci: run gcc-7 on linux-gcc jobs

["Nguyễn Thái Ngọc Duy" <pclouds@gmail.com>]

Switch from gcc-4.8 to gcc-7. Newer compilers come with more warning
checks (usually in -Wextra).  Since -Wextra is enabled in developer
mode (which is also enabled in travis), this lets travis report more
warnings before other people do it.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
---
 .travis.yml        | 3 +++
 ci/lib-travisci.sh | 3 +++
 2 files changed, 6 insertions(+)

diff --git a/.travis.yml b/.travis.yml
index 5f5ee4f3bd..a77f5f5bd5 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -16,10 +16,13 @@ compiler:
 
 addons:
   apt:
+    sources:
+    - ubuntu-toolchain-r-test
     packages:
     - language-pack-is
     - git-svn
     - apache2
+    - gcc-7
 
 matrix:
   include:
diff --git a/ci/lib-travisci.sh b/ci/lib-travisci.sh
index 109ef280da..ef2848fd45 100755
--- a/ci/lib-travisci.sh
+++ b/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-7
+fi
 
 case "$jobname" in
 linux-clang|linux-gcc)
-- 
2.17.0.705.g3525833791