Spoof user agent when downloading GIAS data

Without this the script seems to fail with a forbidden HTTP status code.

```
/usr/local/bundle/ruby/3.4.0/gems/mechanize-2.14.0/lib/mechanize/http/agent.rb:340:in 'Mechanize::HTTP::Agent#fetch': 403 => Net::HTTPForbidden for https://get-information-schools.service.gov.uk/Downloads -- unhandled response (Mechanize::ResponseCodeError)
```

Author: thomasleese

app/lib/mavis_cli/gias/download.rb

@@ -19,6 +19,7 @@ def call
 
         puts "Starting schools data download process..."
         agent = Mechanize.new
+        agent.user_agent_alias = "Mac Safari"
 
         puts "Visiting the downloads page"
         page =