Chore: Fix Coveralls conditional for Travis-CI

Broken when switching from the old matrix to the built in node_js
version

.travis.yml

@@ -49,7 +49,7 @@ script:
   - npm test
 
 after_success:
-  - if [ $TRAVIS_OS_NAME == "linux" ] && [ $NODE_VERSION == "4" ]; then
+  - if [ $TRAVIS_OS_NAME == "linux" ] && [ $TRAVIS_NODE_VERSION == "4" ]; then
       npm run-script coverage;
     fi