Upgrade GCC to gcc-7 for host builds

Author: earlephilhower

.travis.yml

@@ -6,8 +6,56 @@ git:
   depth: 1
   submodules: false
 
+# From https://docs.travis-ci.com/user/languages/cpp/ - to get less ancient GCC/G++
+matrix:
+  include:
+    # works on Precise and Trusty
+    - os: linux
+      addons:
+        apt:
+          sources:
+            - ubuntu-toolchain-r-test
+          packages:
+            - g++-4.9
+      env:
+         - MATRIX_EVAL="CC=gcc-4.9 && CXX=g++-4.9"
+
+    # works on Precise and Trusty
+    - os: linux
+      addons:
+        apt:
+          sources:
+            - ubuntu-toolchain-r-test
+          packages:
+            - g++-5
+      env:
+         - MATRIX_EVAL="CC=gcc-5 && CXX=g++-5"
+
+    # works on Precise and Trusty
+    - os: linux
+      addons:
+        apt:
+          sources:
+            - ubuntu-toolchain-r-test
+          packages:
+            - g++-6
+      env:
+        - MATRIX_EVAL="CC=gcc-6 && CXX=g++-6"
+
+    # works on Precise and Trusty
+    - os: linux
+      addons:
+        apt:
+          sources:
+            - ubuntu-toolchain-r-test
+          packages:
+            - g++-7
+      env:
+        - MATRIX_EVAL="CC=gcc-7 && CXX=g++-7"
+
 before_install:
   - git submodule update --init  # no recursive update
+  - eval "${MATRIX_EVAL}"
 
 cache:
   directories: