[test] support building Lame on travis

https://docs.travis-ci.com/user/languages/javascript-with-nodejs#Node.js-v4-(or-io.js-v3)-compiler-requirements

.travis.yml

@@ -2,5 +2,13 @@ language: node_js
 node_js:
   - "6"
   - "5"
+env:
+  - CXX=g++-4.8
+addons:
+  apt:
+    sources:
+      - ubuntu-toolchain-r-test
+    packages:
+      - g++-4.8
 after_success:
   - "cat ./coverage/lcov.info | ./node_modules/.bin/coveralls"