Changing gem install to upgrade to support gem version upgrades over time

The default behavior that is documented in the cookbook README, i.e. NOT specifying a value for cookbook attribute:

```ruby
[:scout][:version]
```

continues to have the same result as before, i.e. the latest version of the gem present on RubyGems is installed.

However, if the attribute is set to 5.7.2, that is the version that will be installed.

Changing the attribute to 5.7.5 and reconverging a node that was previously configured to use 5.7.2 will result in 5.7.5 being installed as well.