Using brakeman v1.7.1
I noticed that the output_format option was changed to output_formats to support multiple output format types (in commit : 3d55b89)
The included rake task wasn't updated though, so it still uses :output_file, which is ignored. As a result, following the documentation (http://brakemanscanner.org/docs/rake/) while trying to specify an output file results in no file getting generated, a rather perplexing outcome for someone new to the project (as I am)
I've updated the rake file to use the specified output_file as the only output_files argument, and will send you a pull request .