Fixed CI.

.travis.yml

@@ -6,6 +6,12 @@ matrix:
     - compiler: clang
       os: osx
 
+before_install:
+  - if [ "$CXX" == "g++" ]; then sudo add-apt-repository -y ppa:ubuntu-toolchain-r/test; sudo apt-get update -qq; fi
+
+install:
+  - if [ "$CXX" = "g++" ]; then sudo apt-get install -qq g++-4.8; fi
+
 script:
   make test