Try some .travis.yml refactoring

 - Use references to simplify file, add linux allowed failures case

Author: zbeekman

.travis.yml

@@ -24,9 +24,10 @@ matrix:
     - os: osx
       env:
         - BUILD_TYPE="InstallScript"
-          OSX_PACKAGES="gcc@6"
+          OSX_PACKAGES="gcc@6 cmake"
           GCC=6
-    - os: osx
+    - &osx_allowed_failure
+      os: osx
       env:
         - OSX_PACKAGES="gcc cmake"
           GCC=7
@@ -53,7 +54,6 @@ matrix:
             - binutils
             - cmake-data
             - cmake
-#            - shellcheck
     -
       <<: *ubuntu
       env:
@@ -70,7 +70,6 @@ matrix:
             - binutils
             - cmake-data
             - cmake
-#            - shellcheck
     -
       <<: *ubuntu
       env:
@@ -85,13 +84,27 @@ matrix:
             - gcc-6
             - gfortran-6
             - g++-6
-#            - shellcheck
-  allow_failures:
-    - os: osx
+    - &linux_allowed_failure
+      <<: *ubuntu
       env:
-        - OSX_PACKAGES="gcc cmake"
-          GCC=7
+        - GCC=7
           OPENCOARRAYS_DEVELOPER=ON
+      addons:
+        apt:
+          sources:
+            - ubuntu-toolchain-r-test
+            - george-edison55-precise-backports
+          packages:
+            - gcc-7
+            - gfortran-7
+            - g++-7
+            - binutils
+            - cmake-data
+            - cmake
+
+  allow_failures:
+    - *osx_allowed_failure
+    - *linux_allowed_failure
 
 before_install:
   - export FC=gfortran-${GCC}