sinatra: 	rack requires Ruby version >= 2.2.2.

```
ERROR:  Error installing sinatra:
	rack requires Ruby version >= 2.2.2.
```

> Versions of Ruby prior to 2.2.2 are no longer supported as of Sinatra 2.0.
(https://github.com/sinatra/sinatra/blob/d4e3be1e31c631243fcc301909dcd2079548063e/README.md)
