Render asciidoc PDF version only for releases

This commit guards the PDF generation of the asciidoc reference
documentation with a version check.

Since the PDF generation takes a significant amount of time to build,
we should only generate it for non-SNAPSHOT versions.

build.gradle

@@ -158,6 +158,7 @@ configure(rootProject) {
 
 	task('makePDF', type: org.asciidoctor.gradle.AsciidoctorTask) {
 		dependsOn prepareAsciidocBuild
+		onlyIf { !project.version.endsWith("SNAPSHOT") }
 		backends 'pdf'
 		sourceDir "$buildDir/asciidoc/build"
 		sources {