Update imagemmagick checks to support 7.1

On Arch Linux, imagemagick is currently at 7.1.0. I tried increasing the
max version and the version used in the Dockerfile to 7.1 and it appears
to compile and tests pass. Version used for local testing:

```
Linux 5.12.12-arch1-1 #1 SMP PREEMPT Fri, 18 Jun 2021 21:59:22 +0000 x86_64 GNU/Linux

Version: ImageMagick 7.1.0-1 Q16 x86_64 2021-06-21 https://imagemagick.org
Copyright: (C) 1999-2021 ImageMagick Studio LLC
License: https://imagemagick.org/script/license.php
Features: Cipher DPC HDRI Modules OpenMP(4.5)
Delegates (built-in): bzlib cairo djvu fontconfig freetype heic jbig jng jp2 jpeg lcms lqr ltdl lzma openexr pangocairo png raqm raw rsvg tiff webp wmf x xml zip zlib
```

build.rs

@@ -24,7 +24,7 @@ use std::path::{Path, PathBuf};
 use std::process::Command;
 
 const MIN_VERSION: &'static str = "7.0";
-const MAX_VERSION: &'static str = "7.1";
+const MAX_VERSION: &'static str = "7.2";
 
 static HEADER: &'static str = "#include <MagickWand/MagickWand.h>\n";