Loosen mixlib-cli restriction

Can't place `test-kitchen` in a Gemfile with the latest `chef`:

```
Bundler could not find compatible versions for gem "mixlib-cli":
  In Gemfile:
    test-kitchen (>= 0) ruby depends on
      mixlib-cli (~> 1.2.2) ruby

    chef (~> 10.18.0) ruby depends on
      mixlib-cli (1.3.0)
```

I'm guessing that `mixlib-cli` `>= 1.2.0, < 2.0.0` is a legit restriction here.