Update ruby-mini-magick to 4.6.1.

v4.6.1 12 Feb 2017

* Fixed MiniMagick::Image#data to be work for multilayer images where array is
  returned as the JSON representation (@bytheway875)

* Fixed stdout and stderr buffer overload that can happen when processing many
  images using posix-spawn (@lest)

v4.6.0 3 Dec 2016

* Fix Image#exif raising an error when an exif value contains a "=" chracter
* Fix Image#exif raising an error when an exif value spans on multiple lines
* Introduced Image#data as an alternative to Image#details, which uses
  ImageMagick's ability to retrieve identify -verbose output in JSON
  format. This eliminates possibility of any parsing errors. It is available
  on ImageMagick 6.8.8-3 or above.
* Allow Image#format to accept a hash of options as a third argument, which
  will be added to the convert command before original path is added
* Support Pathname in Image.new, as we already supported Pathname in
  Image.open
* Added Tool#stdout which adds - to the command (the same as Tool#stdin does)

(taca)