Maintenance: Update dependency slack-ruby-client to v2.2.0

Gemfile.lock

@@ -214,7 +214,8 @@ GEM
       railties (>= 5.0.0)
     faker (3.2.1)
       i18n (>= 1.8.11, < 2)
-    faraday (2.7.10)
+    faraday (2.7.11)
+      base64
       faraday-net_http (>= 2.0, < 3.1)
       ruby2_keywords (>= 0.0.4)
     faraday-mashify (0.1.1)
@@ -556,7 +557,7 @@ GEM
     simpleidn (0.2.1)
       unf (~> 0.1.4)
     slack-notifier (2.4.0)
-    slack-ruby-client (2.1.0)
+    slack-ruby-client (2.2.0)
       faraday (>= 2.0)
       faraday-mashify
       faraday-multipart