Bump jenkins-pipeline-unit from 1.4 to 1.20

Bumps [jenkins-pipeline-unit](https://github.com/jenkinsci/JenkinsPipelineUnit) from 1.4 to 1.20.
- [Release notes](https://github.com/jenkinsci/JenkinsPipelineUnit/releases)
- [Changelog](https://github.com/jenkinsci/JenkinsPipelineUnit/blob/master/RELEASE.md)
- [Commits](jenkinsci/JenkinsPipelineUnit@v1.4...v1.20)

build.gradle

@@ -35,7 +35,7 @@ dependencies {
   compile group: 'org.assertj',           name: 'assertj-core',   version: '3.4.1'
 
   // Jenkis Pipeline JUnit
-  compile group: 'com.lesfurets', name:'jenkins-pipeline-unit', version: '1.4'
+  compile group: 'com.lesfurets', name:'jenkins-pipeline-unit', version: '1.20'
   compile group: 'com.cloudbees', name: 'groovy-cps',           version: '1.12'
 
   // grapes