test: update node versions, coverage badge

Author: epoberezkin

.travis.yml

@@ -2,7 +2,10 @@ language: node_js
 before_script:
   - git submodule update --init
 node_js:
-  - "0.10"
   - "0.12"
+  - "4"
+  - "5"
+  - "6"
+  - "7"
 after_script:
   - coveralls < coverage/lcov.info

README.md

@@ -3,6 +3,7 @@ Internationalised error messages for [Ajv](https://github.com/epoberezkin/ajv) -
 
 [![Build Status](https://travis-ci.org/epoberezkin/ajv-i18n.svg?branch=master)](https://travis-ci.org/epoberezkin/ajv-i18n)
 [![npm version](https://badge.fury.io/js/ajv-i18n.svg)](http://badge.fury.io/js/ajv-i18n)
+[![Coverage Status](https://coveralls.io/repos/github/epoberezkin/ajv-i18n/badge.svg?branch=master)](https://coveralls.io/github/epoberezkin/ajv-i18n?branch=master)
 
 
 ## Supported locales