Test: Run coveralls when nodejs >= 6 because of its supports

Author: sttk

.travis.yml

@@ -17,7 +17,10 @@ node_js:
   - '0.12'
   - '0.10'
 
-after_success: 'npm run coveralls'
+after_success:
+  - if [ $(echo "${TRAVIS_NODE_VERSION}" | cut -d'.' -f1) -ge 6 ]; then
+      npm run coveralls;
+    fi
 
 os:
   - linux