Advance gcc library version for clang++-7

.travis.yml

diff --git a/.travis.yml b/.travis.yml
index 003d6a1..f48ee1f 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -298,7 +298,7 @@
     addons:
       apt:
         sources: ['ubuntu-toolchain-r-test', 'llvm-toolchain-trusty-7']
-        packages: ['g++-6', 'clang-7', 'ninja-build']
+        packages: ['g++-7', 'clang-7', 'ninja-build']
 
 ################
 # build script #