Update rspec to 2.14.1 and fix deprecation warnings.
Introduce a second adapter: NullDBPostgresAdapter, which includes the special postgres db features of rails 4, including column types and enable_extension.

This adapter is automatically used by NullDB::RSpec::NullifiedDatabase if it detects a test db configured for postgresql.
Enable people to opt out of null-db in a broader nulldb context by only calling #checkpoint! if it's available.
Well, I haven't been able to figure out how to get NullDB.nullify to properly detect postgres, but I can at least allow the caller to declare it on the way in.
Merge remote-tracking branch 'sourcery/master'