Log Message:
Update ruby-pg to 0.21.0.

== v0.21.0 [2017-06-12] Michael Granger <ged@FaerieMUD.org>

Enhancements:
- Move add_dll_directory to the Runtime namespace for newest versions
  of RubyInstaller.
- Deprecate PGconn, PGresult, and PGError top-level constants; a warning
  will be output the first time one of them is used. They will be
  removed in the upcoming 1.0 release.

Documentation fixes:
- Update the docs for PG::Result#cmd_tuples