Require "sentry-sidekiq" gem

Previously this gem was installed to resolve the error of:

```
Warning: GovukError is not configured to track Sidekiq errors, install the sentry-sidekiq gem to track them.
```

in [1].

However this wasn't sufficient because Search API doesn't automatically
require gems, so while the gem was available it wasn't actually used.

By requiring it we can resolve this error.

[1]: #2494

lib/rummager.rb

@@ -25,6 +25,7 @@
 require "plek"
 require "redis-client"
 require "securerandom"
+require "sentry-sidekiq"
 require "sidekiq"
 require "sidekiq-limit_fetch"
 require "statsd"