Update ruby-mini-magick to 4.7.2.

v4.7.2 2017/06/20

Avoid defining methods at runtime whenever a processing method is invoked,
which means that Ruby can keep its method cache, instead of having to clear it
on each processing invocation (thanks to @printercu).

v4.7.1 2017/06/15

Fix errors when calling MiniMagick::Image.open with URLs like
https://pbs.twimg.com/media/DCOD2DXVwAI4xsL.jpg:large, where the : would get
included in the file extension and cause errors with some ImageMagick commands
due to : being a special character to ImageMagick.

(taca)