Fix error on Travis CI
Fix error on Travis CI

Error message:
```
[!] There was an error parsing `Gemfile`: You cannot specify the same gem twice with different version requirements.
You specified: rubocop (< 0.58.0) and rubocop (< 0.51.0). Bundler cannot continue.
```

https://travis-ci.org/emsk/backlog_kit/jobs/465595311#L484

emsk/backlog_kit,emsk/backlog_kit