.travis.yml: switch to GCC 6

After commit a72d1200fb, the Travis-CI
build fails, because we have been using GCC 5 there...  so let's
switch to GCC 6 to test the oldest possible GCC version.

.travis.yml

@@ -9,11 +9,11 @@ matrix:
           sources:
             - ubuntu-toolchain-r-test
           packages:
-            - g++-5
+            - g++-6
             - libcppunit-dev
             - libboost-dev
       env:
-        - MATRIX_EVAL="export CC=gcc-5 CXX=g++-5"
+        - MATRIX_EVAL="export CC=gcc-6 CXX=g++-6"
 
     - os: osx
       osx_image: xcode9.3beta