Drop support of nodejs version < 4 (#18)

Author: s0ph1e

.travis.yml

@@ -5,10 +5,6 @@ node_js:
   - "6"
   - "5"
   - "4"
-  - "0.12"
-  - "0.11"
-  - "0.10"
-  - "iojs"
 
 after_success:
   - travis_retry coveralls < coverage/lcov.info