travis: bump to GCC6

.travis.yml

@@ -14,13 +14,13 @@ addons:
     sources:
     - ubuntu-toolchain-r-test
     packages:
-    - gcc-4.9
-    - g++-4.9
+    - gcc-6
+    - g++-6
     - libgnutls28-dev
     - cmake
 
 before_install:
-  - if [ $TRAVIS_OS_NAME == linux ] && [ $CXX == g++ ]; then export CXX=/usr/bin/g++-4.9 && export CC=/usr/bin/gcc-4.9; fi
+  - if [ $TRAVIS_OS_NAME == linux ] && [ $CXX == g++ ]; then export CXX=/usr/bin/g++-6 && export CC=/usr/bin/gcc-6; fi
   - if [ $TRAVIS_OS_NAME == linux ]; then git clone https://github.com/msgpack/msgpack-c.git; fi
   - if [ $TRAVIS_OS_NAME == linux ]; then cd msgpack-c && git checkout cpp-1.2.0 && mkdir build && mkdir -p $HOME/usr && cd build; fi
   - if [ $TRAVIS_OS_NAME == linux ]; then cmake -DCMAKE_INSTALL_PREFIX=$HOME/usr -DMSGPACK_CXX11=ON -DMSGPACK_BUILD_EXAMPLES=OFF .. && make -j && make install && cd ../.. && pwd; fi