Update dependency pg to v1.5.7 (#367)

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This PR contains the following updates:

| Package | Change | Age | Adoption | Passing | Confidence |
|---|---|---|---|---|---|
| pg | `1.5.6` -> `1.5.7` |
[![age](https://developer.mend.io/api/mc/badges/age/rubygems/pg/1.5.7?slim=true)](https://docs.renovatebot.com/merge-confidence/)
|
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|
[![passing](https://developer.mend.io/api/mc/badges/compatibility/rubygems/pg/1.5.6/1.5.7?slim=true)](https://docs.renovatebot.com/merge-confidence/)
|
[![confidence](https://developer.mend.io/api/mc/badges/confidence/rubygems/pg/1.5.6/1.5.7?slim=true)](https://docs.renovatebot.com/merge-confidence/)
|

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audiences/Gemfile.lock

@@ -139,7 +139,7 @@ GEM
     parser (3.3.4.0)
       ast (~> 2.4.1)
       racc
-    pg (1.5.6)
+    pg (1.5.7)
     psych (5.1.2)
       stringio
     public_suffix (5.0.5)