Update Travis CI compiler version

.travis.yml

@@ -9,7 +9,7 @@ addons:
     sources:
      - ubuntu-toolchain-r-test
     packages:
-     - g++-5
+     - g++-6
      - autotools-dev
      - automake
      - autoconf
@@ -64,7 +64,7 @@ matrix:
       - lsb_release -a
       - uname -a
       - whoami
-      - sudo apt-get install gpm libgpm-dev libcppunit-dev autoconf-archive
+      - sudo apt-get install g++-6 gpm libgpm-dev libcppunit-dev autoconf-archive
       # Work around broken certificate of scan.coverity.com
       - sudo curl -s -L https://entrust.com/root-certificates/entrust_l1k.cer -o /usr/local/share/ca-certificates/entrust_l1k.crt
       - sudo update-ca-certificates