request reviews one at a time

if a batch of review requests contains any users who are not members of the organization, the entire request fails.

ofborg/src/tasks/massrebuilder.rs

@@ -737,18 +737,16 @@ fn request_reviews(
 ) {
     if let Ok(ref maint) = m {
         if maint.maintainers().len() < 10 {
+            for maintainer in maint.maintainers() {
                 if let Err(e) =
                     pull.review_requests()
                         .create(&hubcaps::review_requests::ReviewRequestOptions {
-                        reviewers: maint.maintainers(),
+                            reviewers: vec![maintainer.clone()],
                             team_reviewers: vec![],
                         })
                 {
-                println!(
-                    "Failure adding review requests for maintainers ({:?}): {:#?}",
-                    maint.maintainers(),
-                    e,
-                );
+                    println!("Failure requesting a review from {}: {:#?}", maintainer, e,);
+                }
             }
         }
     }