Update dependency mysql2 to v0.5.6 (#139)

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This PR contains the following updates:

| Package | Change | Age | Adoption | Passing | Confidence |
|---|---|---|---|---|---|
| mysql2 | `0.5.5` -> `0.5.6` |
[![age](https://developer.mend.io/api/mc/badges/age/rubygems/mysql2/0.5.6?slim=true)](https://docs.renovatebot.com/merge-confidence/)
|
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|
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|
[![confidence](https://developer.mend.io/api/mc/badges/confidence/rubygems/mysql2/0.5.5/0.5.6?slim=true)](https://docs.renovatebot.com/merge-confidence/)
|

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Gemfile.lock

@@ -130,7 +130,7 @@ GEM
     mini_portile2 (2.8.5)
     minitest (5.22.2)
     msgpack (1.7.2)
-    mysql2 (0.5.5)
+    mysql2 (0.5.6)
     net-imap (0.3.7)
       date
       net-protocol