conditional-buildstep dep should be optional (#245)

Author: jglick

pom.xml

@@ -75,6 +75,7 @@
       <groupId>org.jenkins-ci.plugins</groupId>
       <artifactId>conditional-buildstep</artifactId>
       <version>1.4.1</version>
+      <optional>true</optional>
     </dependency>
 
     <!-- Workflow plugin imports are used to test compatibility -->