Fallback to db:test:prepare when test:prepare rake is not found (#17)

The rake to prepare rails test in Rails 6 (and below) is `bin/rails db:test:prepare`.

Author: WaKeMaTTa

lib/tasks/jsbundling/build.rake

@@ -6,4 +6,9 @@ namespace :javascript do
 end
 
 Rake::Task["assets:precompile"].enhance(["javascript:build"])
-Rake::Task["test:prepare"].enhance(["javascript:build"]) if Rake::Task.task_defined?("test:prepare")
+
+if Rake::Task.task_defined?("test:prepare")
+  Rake::Task["test:prepare"].enhance(["javascript:build"])
+elsif Rake::Task.task_defined?("db:test:prepare")
+  Rake::Task["db:test:prepare"].enhance(["javascript:build"])
+end