fix: drop Node v6 and update devDeps (#62)

Author: teppeis

.travis.yml

@@ -1,6 +1,5 @@
 language: node_js
 node_js:
-  - "6"
   - "8"
   - "10"
   - "12"

README.md

@@ -118,5 +118,5 @@ If `allowJs: true` in your `tsconfig.json`, assertions in `test/**/*.(js|jsx)` a
 [travis-url]: https://travis-ci.org/power-assert-js/espower-typescript
 [deps-image]: https://david-dm.org/power-assert-js/espower-typescript.svg
 [deps-url]: https://david-dm.org/power-assert-js/espower-typescript
-[node-version]: https://img.shields.io/badge/Node.js%20support-v6,v8,v10,v12-brightgreen.svg
+[node-version]: https://img.shields.io/badge/Node.js%20support-v8,v10,v12-brightgreen.svg
 [license]: https://img.shields.io/npm/l/espower-typescript.svg