Add gcc-8 to Travis.

.travis.yml

@@ -77,7 +77,20 @@ matrix:
           packages:
             - *linux_deps
 
+    - env: PLATFORM=server TOOLS=yes TARGET=release_debug CACHE_NAME=${PLATFORM}-tools-gcc-8 MATRIX_EVAL="CC=gcc-8 && CXX=g++-8"
+      os: linux
+      compiler: gcc-8
+      addons:
+        apt:
+          sources:
+          - sourceline: 'ppa:ubuntu-toolchain-r/test'
+          packages:
+            - gcc-8
+            - g++-8
+            - *linux_deps
+
 before_install:
+  - eval "${MATRIX_EVAL}"
   - if [ "$STATIC_CHECKS" = "yes" ]; then
       unset SCONS_CACHE;
     fi