Add C++11 compiler in travis configuration.

.travis.yml

@@ -1,5 +1,16 @@
 language: node_js
 sudo: false
+compiler:
+  - gcc
+  - clang
+addons:
+  apt:
+    sources:
+    - ubuntu-toolchain-r-test
+    packages:
+    - gcc-4.8
+    - g++-4.8
+    - clang
 node_js:
   - '0.10'
   - '0.12'
@@ -8,6 +19,5 @@ before_install:
   - export DISPLAY=:99.0
   - sh -e /etc/init.d/xvfb start
 install:
+  - if [ "$CXX" = "g++" ]; then export CXX="g++-4.8" CC="gcc-4.8"; fi
   - npm install
-
-