Added a build test for GCC-8 (#3230)

.travis.yml

@@ -73,6 +73,17 @@ matrix:
       env:
         - MATRIX_EVAL="CC=gcc-7 && CXX=g++-7"
         - CONFIGURE_FLAGS="--enable-prere=no --enable-vip=no --enable-buildbot=yes"
+#   Version 8
+    - os: linux
+      addons:
+        apt:
+          sources:
+            - ubuntu-toolchain-r-test
+          packages:
+            - g++-8
+      env:
+        - MATRIX_EVAL="CC=gcc-8 && CXX=g++-8"
+        - CONFIGURE_FLAGS="--enable-prere=no --enable-vip=no --enable-buildbot=yes"
 # Clang
 #   Version 3.9
     - os: linux