Log Message:
Update ruby-pg to 0.18.3.

== v0.18.3 [2015-09-03] Michael Granger <ged@FaerieMUD.org>

Enhancements:
- Use rake-compiler-dock to build windows gems easily.
- Add CI-tests on appveyor and fix test cases accordingly.

Bugfixes:
- Fix data type resulting in wrong base64 encoding.
- Change instance_of checks to kind_of for subclassing. #220
- TextDecoder::Date returns an actual Ruby Date instead of a Time
  (thanks to Thomas Ramfjord)