StashBuildTrigger: Use isBuildable() instead of isDisabled()

The job field should be set at the time of the check.

TODO: Test. What would happen if the trigger tries to start a disabled
job?

Author: proski

src/main/java/stashpullrequestbuilder/stashpullrequestbuilder/StashBuildTrigger.java

@@ -300,9 +300,8 @@ public void run() {
       return;
     }
 
-    AbstractProject project = stashPullRequestsBuilder.getProject();
-    if (project.isDisabled()) {
-      logger.fine(format("Project disabled, skipping build (%s).", project.getName()));
+    if (job.isBuildable()) {
+      logger.fine(format("Job disabled, skipping build (%s).", job.getName()));
       return;
     }