Pin docile so that builds work with Ruby < 2.5 (#141)
Pin docile so that builds work with Ruby < 2.5 (#141)

Due to:

```
In Gemfile:
  coveralls was resolved to 0.8.23, which depends on
    simplecov was resolved to 0.16.1, which depends on
      docile
```

```
Gem::RuntimeRequirementNotMetError: docile requires Ruby version >= 2.5.0.
```
mattbrictson/airbrussh,mattbrictson/airbrussh