Mute bundler warning caused by twice declared gem

```console
$ bundle exec rackup 
Your Gemfile lists the gem tapp (= 1.3.0) more than once.
You should probably keep only one of them.
While it's not a problem now, it could cause errors if you change the version of just one of them later.
```

↑これを黙らせた。

See https://github.com/bundler/bundler/pull/2679