[A] Added gcc-8 to travis

.travis.yml

@@ -58,6 +58,16 @@ matrix:
           sources: ['ubuntu-toolchain-r-test']
           packages: ['g++-7']
 
+    - os: linux
+      sudo: required
+      compiler: gcc
+      env: 
+        - FORT_C_COMPILER=gcc-8
+        - FORT_CXX_COMPILER=g++-8
+      addons:
+        apt:
+          sources: ['ubuntu-toolchain-r-test']
+          packages: ['g++-8']
 
     # OSX / Clang
     - os: osx