Try to use newer compiler on travis for C++11

.travis.yml

@@ -5,6 +5,16 @@ os:
 compiler:
   - gcc
   - clang
+install:
+- if [ "$CXX" = "g++" ]; then export CXX="g++-4.8" CC="gcc-4.8"; fi
+addons:
+  apt:
+    sources:
+    - ubuntu-toolchain-r-test
+    packages:
+    - gcc-4.8
+    - g++-4.8
+    - clang
 branches:
   only:
     - master