Specify ruby version for Netlify
Specify ruby version for Netlify

Most recent build failed because:

```
3:16:59 PM: Bundler could not find compatible versions for gem "ruby":
3:16:59 PM:   In Gemfile:
3:16:59 PM:     ruby
3:16:59 PM:     jekyll-feed (>= 0.11.0) was resolved to 0.11.0, which depends on
3:16:59 PM:       ruby (>= 2.3.0)
```

Which is odd, because the default Ruby version is supposedly 2.3.6 in
Netlify. Make sure it definitely is...

lucymhdavies/lucymhdavies.github.io,lucymhdavies/lucymhdavies.github.io