Need for recent versions of Ruby, specified

after dealing with the problem in #18, I got this error:

```
ruby_dep-1.5.0 requires ruby version >= 2.2.5, ~> 2.2, which is incompatible with the current version, ruby 3.1.2p20
```

2 problems here.

1. Ruby 2.2 is ANCIENT and unsupported. The current stable version is 3.1, and nothing before 2.7 is supported.
2. There's no version of Ruby specified in the repo. It's normal to have a `.ruby-version` file which specifies the version of Ruby that the project was created with.