Travis: add gcc compiler for brunch

.travis.yml

@@ -2,6 +2,14 @@ language: node_js
 node_js:
 - 4
 - 5.8.0
+env:
+  - CXX=g++-4.8
+addons:
+  apt:
+    sources:
+      - ubuntu-toolchain-r-test
+    packages:
+      - g++-4.8
 sudo: false
 deploy:
   provider: npm