Actually fix travis builds

Summary: Apparently we need a C++ compiler!
https://docs.travis-ci.com/user/languages/javascript-with-nodejs#Node.js-v4-(or-io.js-v3)-compiler-requirements
to the rescue.

Test plan:
 - Push to travis
 - See that it succeeds

Auditors: alex

Author: xymostech

Diff for: .travis.yml

@@ -1,4 +1,11 @@
 language: node_js
 node_js:
-- '4'
-- '5'
+  - '4'
+env:
+  - CXX=g++-4.8
+addons:
+  apt:
+    sources:
+      - ubuntu-toolchain-r-test
+    packages:
+      - g++-4.8