cairo: Update version from 1.15.2 to 1.15.6

[Announcement for 1.15.4][1]:

> cairo snapshot 1.15.4 now available
>
> This new snapshot incorporates changes over the past year since the
> 1.15.2 snapshot, including all the fixes from the 1.14 release series.
>
> Of particular note in this snapshot is a wealth of work by Adrian
> Johnson to enhance PDF support, as well as numerous bug fixes provided
> by him and other contributors.
>
> For a complete log of changes since the last release, please see:
>
>     http://cairographics.org/releases/ChangeLog.1.15.4
>
> Features
> --------
> * The PDF backend has gained support for a range of widely used
>   features, including thumbnails, page labels, metadata, document
>   outlines, structured text, hyperlinks, and tags.  Tags permit adding
>   logical info such as headings, tables, figures, etc. that facilitates
>   indexing, accessibility, text reflow, searching, and extraction of the
>   tagged items to other software.  For details on this new PDF
>   functionality, see:
>
>     https://lists.cairographics.org/archives/cairo/2015-June/027427.html
>
> […]

[Announcement for 1.15.6][2]:

> cairo snapshot 1.15.6 now available
>
> This new snapshot incorporates changes over the past half-year since the
> 1.15.4 snapshot, including all the fixes from the 1.14 release series.
>
> The PDF code continues to be enhanced, and we're restored MacOSX 10.4
> support.  Font-related fixes and improved error handling for X round out
> the release.
>
> For a complete log of changes, please see
>
>     http://cairographics.org/releases/ChangeLog.1.15.6
>
>
> Features and Enhancements
> -------------------------
> * Detect if variable fonts have synthesized bold/italic or non-default
>   variants, and use a fallback font where needed.
>
> * Restore MacOSX 10.4 support.  Cairo had dropped 10.4 support when
>   moving to the CoreText API.  Now we automatically detect which API to
>   use via dynamic linking, so can resume supporting this older version
>   of MacOSX.
>
> […]

[1]: https://www.cairographics.org/news/cairo-1.15.4/
[2]: https://www.cairographics.org/news/cairo-1.15.6/

cairo.be0

@@ -1,6 +1,6 @@
 #!/usr/bin/env beesh
 
-# BEE_VERSION cairo-1.15.2-0
+# BEE_VERSION cairo-1.15.6-0
 
 ## this file was created by bee init and should be executed to build a
 ## bee-package. (Additional hints are located at the end of this file.)