Log Message:
Update ruby-mini-magick to 4.8.0.

v4.8.0								2017/07/06

* Add options to MiniMagick::Image.open which are forwarded to open-uri when
  URL is used (@acrogenesis)

* Fixed MiniMagick::Image#get_pixels not returning all pixels for images that
  have first or last bytes that could be interpreted as control characters in
  their RGB output (@LAndreas)