Update pg requirement from >= 0.18, <= 1.5 to >= 0.18, <= 1.6

Updates the requirements on [pg](https://github.com/ged/ruby-pg) to permit the latest version.
- [Release notes](https://github.com/ged/ruby-pg/releases)
- [Changelog](https://github.com/ged/ruby-pg/blob/master/History.md)
- [Commits](ged/ruby-pg@v0.18.0...v1.5.1)

---
updated-dependencies:
- dependency-name: pg
  dependency-type: direct:development
...

Signed-off-by: dependabot[bot] <[email protected]>

statesman.gemspec

@@ -26,7 +26,7 @@ Gem::Specification.new do |spec|
   spec.add_development_dependency "bundler", "~> 2"
   spec.add_development_dependency "gc_ruboconfig", "~> 3.6.0"
   spec.add_development_dependency "mysql2", ">= 0.4", "< 0.6"
-  spec.add_development_dependency "pg", ">= 0.18", "<= 1.5"
+  spec.add_development_dependency "pg", ">= 0.18", "<= 1.6"
   spec.add_development_dependency "rails", ">= 5.2"
   spec.add_development_dependency "rake", "~> 13.0.0"
   spec.add_development_dependency "rspec", "~> 3.1"