Disable Gradle reference task on the Travis

* Use `NO_REFERENCE_TASK=true` ENV var to skip `reference` tasks
* To enable `reference` tasks directly even with `NO_REFERENCE_TASK=true` use `ignoreEnvToStopReference` project (`-P`) variable

.travis.yml

@@ -1,11 +1,14 @@
 language: java
 jdk: oraclejdk8
 sudo: false
+cache:
+  directories:
+  - $HOME/.gradle
 services:
   - mongodb
   - rabbitmq
   - redis-server
 env:
-  - TERM=dumb SI_FATAL_WHEN_NO_BEANFACTORY=true
+  - TERM=dumb SI_FATAL_WHEN_NO_BEANFACTORY=true NO_REFERENCE_TASK=true
 #script:
-#  - ./gradlew build --parallel
+#  - travis_wait ./gradlew build

build.gradle

@@ -786,6 +786,10 @@ reference {
 
 reference.dependsOn asciidoctor
 
+[asciidoctor, reference, referenceEpub, referenceHtmlMulti, referenceHtmlSingle, referencePdf].each {
+	it.onlyIf { "$System.env.NO_REFERENCE_TASK" != 'true' || project.hasProperty('ignoreEnvToStopReference') }
+}
+
 apply plugin: 'sonar-runner'
 
 sonarRunner {