Maintenance: Update dependency slack-ruby-client to v2.5.2

Gemfile.lock

@@ -695,7 +695,7 @@ GEM
     simple_po_parser (1.1.6)
     simpleidn (0.2.3)
     slack-notifier (2.4.0)
-    slack-ruby-client (2.5.1)
+    slack-ruby-client (2.5.2)
       faraday (>= 2.0)
       faraday-mashify
       faraday-multipart