Bump pg from 0.20.0 to 0.21.0
Bump pg from 0.20.0 to 0.21.0

Bumps [pg](https://bitbucket.org/ged/ruby-pg) from 0.20.0 to 0.21.0.
- [Changelog](https://bitbucket.org/ged/ruby-pg/src/master/History.rdoc)
- [Commits](https://bitbucket.org/ged/ruby-pg/branches/compare/v0.21.0..v0.20.0)
rdunlop/unicycling-rulebook,rdunlop/unicycling-rulebook,rdunlop/unicycling-rulebook,rdunlop/unicycling-rulebook