Log Message:
Update ruby-mini-magick to 4.2.7.

4.2.7

* Fix Ruby 1.9.3

4.2.5

* Fixed MiniMagick::Image#details parsing

4.2.4

* Fixed detecting MiniMagick version
* Fix the private method 'format' called on mogrify for real

4.2.3

* Fixed a rush mistake

4.2.2

* Raise proper error in #cheap_info on invalid image
* Improved MiniMagick::Image#details parsing
* Attempt to solve private method 'format' called on mogrify

4.2.1

* Attempt to fix #279

4.1.1

* Fix MiniMagick::Image#composite defaulting to JPG extension. The default is
  now the extension of the base image, the one on which composite is called.