Increase Rubocop version since guard-rubocop requires 0.27.1.

When I ran `bundle` on `v2.0.0` branch I got this error:
```bash
admin@shein ~/projects/active_interaction (v2.0.0=) $ bundle
Fetching gem metadata from https://rubygems.org/.........
Resolving dependencies...
You have requested:
  rubocop = 0.26.0

The bundle currently has rubocop locked at 0.27.1.
Try running `bundle update rubocop`
```
This pull fixes it.