Handle frame-src/child-src gracefully

3.4.0 the frame-src/child-src transition for Firefox.

Handle the child-src/frame-src transition semi-intelligently across versions. I think the code best describes the behavior here:

if supported_directives.include?(:child_src)
  @config[:child_src] = @config[:child_src] || @config[:frame_src]
else
  @config[:frame_src] = @config[:frame_src] || @config[:child_src]
end