[LTP] [PATCH v2] travis: Use centos:7 instead of centos:6

[Yang Xu]

I have this idea when I meet ALG_SET_AEAD_ASSOCLEN undefined error on centos7.
It is more friendly for centos7 user and more meaningful. centos6 is outdated
and we should drop it.

Signed-off-by: Yang Xu <xuyang2018.jy@cn.fujitsu.com>
---
 .travis.yml | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/.travis.yml b/.travis.yml
index e8e3189af..3c5ff99b7 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -44,7 +44,7 @@ matrix:
           compiler: clang
 
         - os: linux
-          env: DISTRO=centos:6 TREE=out
+          env: DISTRO=centos:7 TREE=out
           compiler: gcc
 
         - os: linux
-- 
2.23.0