Refactor ENV stubbing in zendesk_sender_spec

Upgrading to version 3 of the dotenv-rails gem causes tests in
zendesk_sender_spec to fail with the error `FrozenError: can't modify
frozen Hash: [...]`.

This is because the latest version of dotenv-rails introduces an
Autorestore feature which automatically restores the state of ENV
between rspec tests. In zendesk_sender_spec we stub the value of
ENV['ENV'] using `stub_const`, which also resets between tests and leads
to a conflict: bkeepers/dotenv#482.

This is resolved by replacing the use of `stub_const` with the
`Dotenv.modify` method (also introduced in v3), which makes dotenv-rails
fully responsible for the stubbing of environment variables and avoids
conflict.

spec/services/zendesk_sender_spec.rb

@@ -28,9 +28,12 @@
       }
     end
 
-    before do
-      stub_const('ENV', ENV.to_hash.merge('ENV' => 'test_environment'))
-      allow(ZendeskAPI::Ticket).to receive(:create!)
+    before { allow(ZendeskAPI::Ticket).to receive(:create!) }
+
+    around do |example|
+      Dotenv.modify(ENV: 'test_environment') do
+        example.run
+      end
     end
 
     it 'calls ZendeskAPI::Ticket.create!' do