fix(deps): update dependency err-backend-discord to v4 (#73)

This PR contains the following updates:

| Package | Change | Age | Adoption | Passing | Confidence |
|---|---|---|---|---|---|
| err-backend-discord | `==3.0.1` -> `==4.0.0` |
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|
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setup.py

@@ -108,7 +108,7 @@ def read(fname, encoding="ascii"):
                 "errbot-backend-slackv3==0.3.1",
             ],
             "discord": [
-                "err-backend-discord==3.0.1",
+                "err-backend-discord==4.0.0",
             ],
             "mattermost": [
                 "err-backend-mattermost==3.0.0",