message: chore(ci): add badge for Travis CI
diff: [![npm version](https://badge.fury.io/js/%40kintone%2Fwebpack-plugin-kintone-plugin.svg)](https://badge.fury.io/js/%40kintone%2Fwebpack-plugin-kintone-plugin)
  [![CircleCI](https://circleci.com/gh/kintone/webpack-plugin-kintone-plugin.svg?style=shield)](https://circleci.com/gh/kintone/webpack-plugin-kintone-plugin)
  -[![Build status](https://ci.appveyor.com/api/projects/status/9wc2rotaxfgtdyy8/branch/master?svg=true)](https://ci.appveyor.com/project/cybozu-frontend/webpack-plugin-kintone-plugin/branch/master)
  +[![Build Status](https://travis-ci.org/kintone/webpack-plugin-kintone-plugin.svg?branch=master)](https://travis-ci.org/kintone/webpack-plugin-kintone-plugin)
  A webpack plugin to create a plugin zip of kintone.
  