Remove unnecessary Ruby version specification on CI
Remove unnecessary Ruby version specification on CI

This commit removes unnecessary Ruby version specification of
Docker image on CircleCI config.

The latest version is used if version is not specified,

```console
% docker pull circleci/ruby
Using default tag: latest
latest: Pulling from circleci/ruby
(snip)
```

With this change we don't have to update the version.

meganemura/rubocop,tejasbubane/rubocop,rrosenblum/rubocop,palkan/rubocop,meganemura/rubocop,bquorning/rubocop,mikegee/rubocop,jfelchner/rubocop,tdeo/rubocop,maxjacobson/rubocop,smakagon/rubocop,panthomakos/rubocop,bquorning/rubocop,bbatsov/rubocop,palkan/rubocop,deivid-rodriguez/rubocop,tejasbubane/rubocop,panthomakos/rubocop,tejasbubane/rubocop,mikegee/rubocop,palkan/rubocop,jfelchner/rubocop,tdeo/rubocop,mikegee/rubocop,panthomakos/rubocop,jmks/rubocop,vergenzt/rubocop,bbatsov/rubocop,jmks/rubocop,meganemura/rubocop,tdeo/rubocop,deivid-rodriguez/rubocop,vergenzt/rubocop,maxjacobson/rubocop,vergenzt/rubocop,deivid-rodriguez/rubocop,rrosenblum/rubocop,jmks/rubocop,bquorning/rubocop,jfelchner/rubocop,maxjacobson/rubocop,smakagon/rubocop,rrosenblum/rubocop