Merge #6703

6703: Switch from Homu to Bors mergebots r=colby-swandale a=indirect

### What was the end-user problem that led to this PR?

Our existing @bundlerbot mergebot is run by [bundlerbot-homu](https://github.com/bundler/bundlerbot-homu), which is a fork of japaric/homu-on-heroku, which is a fork of servo/homu, which is a fork of barosl/homu. Sadly, most of those forks are now unmaintained, including ours. Also, Homu sets up a list of users who are allowed to approve PRs with a list in a file on an env var, when it probably should just read the list of github users that have the permission to merge.

So the end-user problem is: unmaintained mergebot, very inconvenient to update team members.

### What was your diagnosis of the problem?

My diagnosis is that we should migrate from a Homu-based mergebot to a bors-ng-based mergebot. Bors-ng lives at [bors-ng/bors-ng](https://github.com/bors-ng/bors-ng), and has a website at [bors.tech](https://bors.tech). Bors-ng is actively maintained, easy to deploy to Heroku, doesn't lose repo and PR state when the webapp is restarted, and uses GitHub permissions to determine who can approve PRs for merging.

### What is your fix for the problem, implemented in this PR?

My fix for this problem was to fork bors-ng to the @bundler organization, modify bors-ng to respond to `@bundlerbot r+` instead of responding to `bors r+`, and change the configuration files below to support using bors instead of homu.

The bors-ng repo has already accepted our patch, and we maybe don't even need to keep the forked repo anymore.

The new mergebot lives at https://bundlerbot-bors.herokuapp.com, and has the exact same issue-comment interface as the previous mergebot.

### Why did you choose this fix out of the possible options?

I chose this fix because it lets us keep the exact same mergebot interface that we have today, while easing the maintenance burden both on updating end-user permissions and on updating the app over time, since we can simply pull and deploy the latest from `bors-ng/bors-ng` without maintaining our own specialized fork.

Co-authored-by: Andre Arko <andre@arko.net>

.github/bors.toml

@@ -0,0 +1,9 @@
+status = [
+  "continuous-integration/travis-ci/push"
+]
+
+timeout-sec = 14400
+
+[committer]
+name = "Bundlerbot"
+email = "bot@bundler.io"

.travis.yml

@@ -9,7 +9,8 @@ before_script:
 branches:
   only:
     - master
-    - auto
+    - staging
+    - trying
     - /.+-dev$/
     - /.+-stable$/