Log Message:
Update ruby-mini-magick to 4.7.0.

4.7.0								2017/03/28

* Added MiniMagick::Image#get_pixels, which returns a matrix where each member
  is a 3-element array of numbers between 0 and 255, one for each of the RGB
  channels.

* When MiniMagick.timeout is set and the command times out, previously the
  command would still continue running in the background. Now when
  Timeout::Error is raised, we also kill the subprocess running the command
  with SIGTERM.

* Implementation of posix-spawn has been improved, where now both stdout and
  stderr are read from at the same time, stdin pipe is closed immediately
  after writing the input, and stdout and stderr pipes are closed once the
  command finishes. This now has essentially the same behaivour as
  Open3.popen3 with a block.