travis: add a test that we compile under gcc-4.8 (CentOS!)

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

.travis.yml

@@ -14,6 +14,7 @@ addons:
       - cppcheck
       - valgrind
       - libomp-dev
+      - gcc-4.8
 
 env:
   - ARCH=64 SOURCE_CHECK_ONLY=true CDEBUGFLAGS="-std=gnu11 -g -fstack-protector -O3 -flto" LDFLAGS="-O3 -flto"
@@ -25,6 +26,7 @@ env:
   - VALGRIND=1 ARCH=64 DEVELOPER=0 COMPILER=gcc TEST_GROUP=2 TEST_GROUP_COUNT=3 SOURCE_CHECK_ONLY=false
   - VALGRIND=1 ARCH=64 DEVELOPER=0 COMPILER=gcc TEST_GROUP=3 TEST_GROUP_COUNT=3 SOURCE_CHECK_ONLY=false
   - VALGRIND=0 ARCH=64 DEVELOPER=1 COMPILER=clang SOURCE_CHECK_ONLY=false
+  - VALGRIND=0 ARCH=64 DEVELOPER=1 COMPILER=gcc-4.8 SOURCE_CHECK_ONLY=false
   - VALGRIND=1 ARCH=64 DEVELOPER=1 COMPILER=gcc TEST_GROUP=1 TEST_GROUP_COUNT=6 SOURCE_CHECK_ONLY=false
   - VALGRIND=1 ARCH=64 DEVELOPER=1 COMPILER=gcc TEST_GROUP=2 TEST_GROUP_COUNT=6 SOURCE_CHECK_ONLY=false
   - VALGRIND=1 ARCH=64 DEVELOPER=1 COMPILER=gcc TEST_GROUP=3 TEST_GROUP_COUNT=6 SOURCE_CHECK_ONLY=false