Fix incorrect version specification of `bunny`

Without this fix, `bundle update` raise an error as follows.

```
Bundler could not find compatible versions for gem "bunny":
  In snapshot (Gemfile.lock):
    bunny (= 2.6.7)

  In Gemfile:
    sneakers was resolved to 2.7.0, which depends on
      bunny (~> 2.9.2)

Running `bundle update` will rebuild your snapshot from scratch, using only
the gems in your Gemfile, which may resolve the conflict.
```

It seems that made an incorrect version specification
with 655ff935.

Gemfile.lock

@@ -162,8 +162,8 @@ GEM
     bootsnap (1.2.1-java)
       msgpack (~> 1.0)
     builder (3.2.3)
-    bunny (2.6.7)
-      amq-protocol (~> 2.2.0)
+    bunny (2.9.2)
+      amq-protocol (~> 2.3.0)
     byebug (10.0.2)
     capybara (3.0.1)
       addressable