Update Slave Installer Module to 1.6

* [PR #1](https://github.com/jenkinsci/slave-installer-module/pull/1/) - Cleanup issues reported by FisndBugs
* [JENKINS-42841](https://issues.jenkins-ci.org/browse/JENKINS-42841) - Replace the "slave" term by "agent" where possible

Changelog: https://github.com/jenkinsci/slave-installer-module/blob/master/CHANGELOG.md#16

war/pom.xml

@@ -109,7 +109,7 @@ THE SOFTWARE.
     <dependency>
       <groupId>org.jenkins-ci.modules</groupId>
       <artifactId>slave-installer</artifactId>
-      <version>1.5</version>
+      <version>1.6</version>
     </dependency>
     <dependency>
       <groupId>org.jenkins-ci.modules</groupId>