add right compiler to travis conf

attempt n°2  to fix travis build

.travis.yml

@@ -5,10 +5,17 @@ node_js:
   - "4.0.0"
 env:
   - NODE_ENV=travis
+  - TRAVIS=travis CXX=g++-4.8
 services:
   - mongodb
 addons:
   code_climate:
     repo_token: 6c3a1b81a09b2338d6f30913c1bcad115026689752cbb499a0a25061cda6fbcf
+  apt:
+    sources:
+    - ubuntu-toolchain-r-test
+    packages:
+    - gcc-4.8
+    - g++-4.8
 after_script:
   - grunt coverage