Log Message:
Update ruby-pg to 0.20.0

== v0.20.0 [2017-03-10] Michael Granger <ged@FaerieMUD.org>

Enhancements:
- Update error codes to PostgreSQL-9.6
- Update Windows binary gems to Ruby-2.4, PostgreSQL 9.6.1 and
  OpenSSL 1.0.2j.
- Add support for RubyInstaller2 to Windows binary gems.

Bugfixes:
- Use secure JSON methods for JSON (de)serialisation. #248
- Fix Result#inspect on a cleared result.
- Fix test case that failed on Ruby-2.4. #255

Documentation fixes:
- Talk about Integer instead of Fixnum.
- Fix method signature of Coder#encode.