Fix ImageMagick 6/7 compatibility

[alessandroberna]

Unfortunately the issue is still not fixed in the latest release. With ImageMagick 7 the warning gets printed both when using convert and magick convert:

Since the correct way to issue the convert command seems to be using magick without the tool name, simply appending magick in front of the old command does not resolve the issue:

I tried to keep the changes minimal:

• Introduced a convert variable that is set to $magick if it exists, or to convert otherwise.
• Updated lines that used $magick convert to use $convert.

There's no brittle version parsing and the fix does not depend on other standalone tools that may be deprecated in the future. I also made sure that the non-sixel error message prints the correct ImageMagick command.

I had to create a new PR as I did not have the permissions to re-open #73. I hope this PR addresses the issues more effectively.

[hackerb9]

Thank you for filing a new pull request. I'll have to talk with the ImageMagick folks and see what their intention is for other breaking changes.