Bump pg from 1.5.3 to 1.5.6

Bumps [pg](https://github.com/ged/ruby-pg) from 1.5.3 to 1.5.6.
- [Changelog](https://github.com/ged/ruby-pg/blob/master/History.md)
- [Commits](ged/ruby-pg@v1.5.3...v1.5.6)

---
updated-dependencies:
- dependency-name: pg
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

Gemfile.lock

@@ -242,7 +242,7 @@ GEM
     parser (3.3.3.0)
       ast (~> 2.4.1)
       racc
-    pg (1.5.3)
+    pg (1.5.6)
     psych (5.1.2)
       stringio
     public_suffix (5.0.5)