Redirect dangerous_eval to correct page

[BlakeWilliams]

This adds a redirect for:

http://brakemanscanner.org/docs/warning_types/dangerous_eval

to:

http://brakemanscanner.org/docs/warning_types/dangerous_evaluation/

This is necessary because the brakeman gem points to this url as the link for the dangerous eval rule.

[BlakeWilliams]

I also considered making /dangerous_eval the correct URL, but there was already one redirect pointing to /dangerous_evaluation so I made /dangerous_eval redirect instead. Let me know if we prefer it the other way around.

[presidentbeef]

It would be better the other way. As you can see, this isn't the first time I got the warning->URL mapping wrong.

[BlakeWilliams]

@presidentbeef Updated.

[presidentbeef]

Looks good, thanks!