Log Message:
Update ruby-pg to 0.18.1.

== v0.18.1 [2015-01-05] Michael Granger <ged@FaerieMUD.org>

Correct the minimum compatible Ruby version to 1.9.3. #199

== v0.18.0 [2015-01-01] Michael Granger <ged@FaerieMUD.org>

Bugfixes:
- Fix OID to Integer mapping (it is unsigned now). #187
- Fix possible segfault in conjunction with notice receiver. #185

Enhancements:

- Add an extensible type cast system.
- A lot of performance improvements.
- Return frozen String objects for result field names.
- Add PG::Result#stream_each and #stream_each_row as fast helpers for
  the single row mode.
- Add Enumerator variant to PG::Result#each and #each_row.
- Add PG::Connection#conninfo and #hostaddr.
- Add PG.init_openssl and PG.init_ssl methods.
- Force zero termination for all text strings that are given to libpq.
  It raises an ArgumentError if the string contains a null byte.
- Update Windows cross build to PostgreSQL 9.3.