Support Imagemagick 7

[eggplants]

Is your feature request related to a problem? Please describe.

Related: #140

kt-paperclip raises Paperclip::Errors::CommandNotFoundError in ImageMagick 7.1.1-43.

Describe the solution you'd like
Invoke magick convert / magick identify from Paperclip::Processor#{convert,identify} if imagemagick 7 or above is installed.

Describe alternatives you've considered
None.