Add Security/Open rubocop rule

The `open` method can be used to open files or URLs and it's deprecated
in Ruby 2.7. In this case, it's clear we're dealing with a URL, so we
can use `URI.parse`.

The code was a bit strange, since it returned a value and had a side
effect: opening the URL. I'm not sure about the intention of the code;
my best guess is we wanted to test the URL exists and was accessible
before returning it (and, if that's the case, IMHO the code should be a
bit more explicit in order to show the intention behind it), but it
could also be an unintended side effect which was there by accident.

Now the URL is no longer opened; if the URL isn't accessible, we'll find
out when trying to connect to it with the Savon client.

.rubocop.yml

@@ -446,6 +446,9 @@ Security/Eval:
 Security/JSONLoad:
   Enabled: true
 
+Security/Open:
+  Enabled: true
+
 Security/YAMLLoad:
   Enabled: true
 

lib/sms_api.rb

@@ -9,7 +9,7 @@ class SMSApi
   def url
     return "" unless end_point_available?
 
-    open(Rails.application.secrets.sms_end_point).base_uri.to_s
+    URI.parse(Rails.application.secrets.sms_end_point).to_s
   end
 
   def authorization