Use GCC 7 for coverage

Author: d-frey

.travis.yml

@@ -1,5 +1,9 @@
 language: cpp
 
+branches:
+  only:
+    - master
+
 matrix:
   include:
     - os: linux
@@ -147,13 +151,14 @@ matrix:
         - COMPILER=clang++
 
     - os: linux
+      dist: trusty
       compiler: gcc
       addons:
         apt:
           sources: ['ubuntu-toolchain-r-test']
-          packages: ['g++-6']
+          packages: ['g++-7']
       env:
-        - COMPILER=g++-6
+        - COMPILER=g++-7
         - CXXFLAGS="-O0 --coverage"
       before_script:
         - pip install --user urllib3[secure] cpp-coveralls
@@ -162,7 +167,7 @@ matrix:
         # https://urllib3.readthedocs.io/en/latest/user-guide.html#ssl-py2
         - sed -i -e '/^import sys$/a import urllib3.contrib.pyopenssl\nurllib3.contrib.pyopenssl.inject_into_urllib3()' $(which coveralls)
       after_success:
-        - coveralls --gcov gcov-6 --gcov-options '\-lp' --exclude src --exclude include/tao/json/external
+        - coveralls --gcov gcov-7 --gcov-options '\-lp' --exclude src --exclude include/tao/json/external
 
     - os: linux
       dist: trusty