Update installation instructions for Ruby 2.4.9

This is the Ruby version CONSUL uses now. Thanks to that, we no longer
need to install an old version of libssl-dev for Ubuntu 18.04.

We could probably simplify the documentation using:

```bash
rbenv install `cat .ruby_version`
```

However, this would require downloading consul before installing Ruby,
which could be harder to document.

docs/en/installation/deploying-on-heroku.md

@@ -119,7 +119,7 @@ As recommended by Heroku, you can add the gem rails\_12factor and specify the ve
 ```
 gem 'rails_12factor'
 
-ruby '2.3.2'
+ruby '2.4.9'
 ```
 
 in the file _Gemfile\_custom_. Don't forget to run