Load dependencies on demand, add brakeman-min gemspec

Currently, Brakeman loads most dependencies up front, even if they aren't needed (e.g., HAML and Slim for apps that only use ERB). In order to support a gem with minimal dependencies, these are now loaded only as needed.

The brakeman-min gem is probably not going to be useful on its own, but will allow users to only install the gems they need. Since `terminal-table` and `highline` are required for the regular text output, Brakeman will now output JSON by default *if* the `terminal-table` gem is not available.

This reduces gem dependencies for brakeman-min from 13 gems to 4, which seems pretty good.