Remove Metarequest gem and add byebug gem.

When files were uploaded with Paperclip an exception for undefined
conversion "\xFF" from ASCII-8BIT to UTF-8 was thrown.  Historically,
Metarequest was used little to develop this project, and so it can be
removed without any ill effects.  The byebug gem and the console
debugger was used to track down this interaction, so byebug is added to
the development gem group.
Add content type validation to blog_image model.

Add content type validation to blog_image model.  This is required by
recent versions of the Paperclip gem.  It assures that the file which is
uploaded is of the same type that is expected.  Useful to help prevent
an attacker from uploading malicious files which can damage any clients
who may download them.
Update strong parameters in blog_posts_controller.

Set blog_images_array to an empty array in blog_post_params.  This is
needed for Rails 4 and the nesting of blog_image attributes in the
blog_post model.