Upgrade AsciidoctorJ to version 2.5.7

This fixes a library loading issue on Apple's M1 hardware.
The problem is caused by a version of JRuby that does not
support macos-aarch64. By upgrading AsciidoctorJ, a newer
version of JRuby supporting M1 is used.

Author: jmp

gradle.properties

@@ -7,7 +7,7 @@ issues                      = https://github.com/jbake-org/jbake/issues
 vcs                         = https://github.com/jbake-org/jbake/
 
 # runtime dependencies
-asciidoctorjVersion         = 2.5.2
+asciidoctorjVersion         = 2.5.7
 asciidoctorjDiagramVersion  = 2.2.1
 args4jVersion               = 2.33
 commonsIoVersion            = 2.11.0