Use code block switch extension from separate project

This should have been merged forwards following the changes in
70df55d413.

See gh-229

docs/build.gradle

@@ -2,7 +2,14 @@ plugins {
 	id 'org.asciidoctor.convert' version '1.5.3'
 }
 
+repositories {
+	maven {
+		url 'https://repo.spring.io/release'
+	}
+}
+
 dependencies {
+	asciidoctor 'io.spring.asciidoctor:spring-asciidoctor-extensions:0.1.0.RELEASE'
 	testCompile project(':spring-restdocs-mockmvc')
 	testCompile project(':spring-restdocs-restassured')
 	testCompile 'io.rest-assured:rest-assured'