[A] Adding gcc-9 to tests

.travis.yml

@@ -97,6 +97,18 @@ matrix:
           sources: ['ubuntu-toolchain-r-test']
           packages: ['g++-8']
 
+    - name: "Linux gcc-9"
+      os: linux
+      sudo: required
+      compiler: gcc
+      env:
+        - FORT_C_COMPILER=gcc-9
+        - FORT_CXX_COMPILER=g++-9
+      addons:
+        apt:
+          sources: ['ubuntu-toolchain-r-test']
+          packages: ['g++-9']
+
 
     # OSX / Clang
     - name: "OSX xcode7.3"