Use asciidoctor 1.5.8

This commit uses asciidoctor 1.5.8 for the time being. Once a new
version of the plugin is released, this override will be unnecessary.

spring-boot-project/spring-boot-parent/pom.xml

@@ -294,6 +294,13 @@
 					<groupId>org.asciidoctor</groupId>
 					<artifactId>asciidoctor-maven-plugin</artifactId>
 					<version>1.5.7.1</version>
+					<dependencies>
+						<dependency>
+							<groupId>org.asciidoctor</groupId>
+							<artifactId>asciidoctorj</artifactId>
+							<version>1.5.8</version>
+						</dependency>
+					</dependencies>
 				</plugin>
 				<plugin>
 					<groupId>org.apache.maven.plugins</groupId>