Update Travis configuration to use GCC 8

.travis.yml

@@ -4,6 +4,8 @@ sudo: false
 
 addons:
   apt:
+    sources:
+      - ubuntu-toolchain-r-test
     packages:
       - bison
       - flex
@@ -12,6 +14,7 @@ addons:
       - debhelper
       - dkms
       - fakeroot
+      - gcc-8
 
 env:
   - KVER=4.4
@@ -22,9 +25,9 @@ env:
   - KVER=4.15
   - KVER=4.16
   - KVER=4.17
-  - KVER=5.0
-  - KVER=5.1
-  - KVER=master
+  - KVER=5.0 && CC=gcc-8
+  - KVER=5.1 && CC=gcc-8
+  - KVER=master && CC=gcc-8
 
 matrix:
   allow_failures: