Test with matrix-auth plugin 3.1.8 (#79)

Do not need to use the incremental build for this test.

Using the incremental build was blocking the release of the plugin
because the release correctly requires that all dependencies must be
released dependencies, not incrementals.

pom.xml

@@ -69,7 +69,7 @@
       <!-- Needed by JTH -->
       <groupId>org.jenkins-ci.plugins</groupId>
       <artifactId>matrix-auth</artifactId>
-      <version>3.2-rc393.c200fd32fcd4</version>
+      <version>3.1.8</version>
       <scope>test</scope>
     </dependency>
   </dependencies>