Check Rails.backtrace_cleaner method exists (#734)

Check if the Rails.backtrace_cleaner method exists before calling it.
We've got a report from a user that the method call fails with a
NoMethodError in a Sinatra app using the activesupport and actionmailer
gems version 5.2.0.

```
NoMethodError - undefined method `backtrace_cleaner' for Rails:Module:
/gems/appsignal-3.0.6/lib/appsignal/transaction.rb:530:in `cleaned_backtrace'
```

I was unable to reproduce in the test setups, but I can see a scenario
where this could fail. To make sure we don't cause this kind of error in
such a scenario, check if the Rails module listens to the
`backtrace_cleaner` method before calling it.

appsignal/test-setups@d484b36

.changesets/check-rails-backtrace_cleaner-method.md

@@ -0,0 +1,5 @@
+---
+bump: "patch"
+---
+
+Check Rails.backtrace_cleaner method before calling the method. This prevents a NoMethodError from being raised in some edge cases.

lib/appsignal/transaction.rb

@@ -526,7 +526,7 @@ def sanitized_tags
     end
 
     def cleaned_backtrace(backtrace)
-      if defined?(::Rails) && backtrace
+      if defined?(::Rails) && Rails.respond_to?(:backtrace_cleaner) && backtrace
         ::Rails.backtrace_cleaner.clean(backtrace, nil)
       else
         backtrace

spec/lib/appsignal/transaction_spec.rb

@@ -1321,6 +1321,13 @@ def session_exists?(_env)
         expect(subject).to eq ["line 1", "line 2"]
       end
 
+      context "with Rails module but without backtrace_cleaner method" do
+        it "returns the backtrace uncleaned" do
+          stub_const("Rails", Module.new)
+          expect(subject).to eq ["line 1", "line 2"]
+        end
+      end
+
       if rails_present?
         context "with rails" do
           it "cleans the backtrace with the Rails backtrace cleaner" do