Replaces any non-alphanumeric characters with '-' when creating JUnit re...

...porter filenames.

Fixes a crash when using JUnit Reporters with a `describe` that has a '/' in it.

I didn't add a test for this since there aren't any other reporter-specific tests, but I have a little repo that shows the problem here: https://github.com/carols10cents/junit-reporter-bug

I basically copied what [ci-reporter does](https://github.com/nicksieger/ci_reporter/blob/master/lib/ci/reporter/report_manager.rb#L40) to prevent slashes and other problematic characters from being used in filenames.

Let me know if there's anything else I can do for this. Thanks! :heart: