Setting up PostgreSQL with Ruby 2.3, you may encounter this error:
LoadError: cannot load such file -- 2.3/pg_ext
This is due to the native extensions being built for Ruby 2.2 instead.
How I solved this:
Uninstall the pg gem
gem uninstall pg
Instead of pg gem v0.18 install a pre-release version of v0.19
gem install pg --pre
In your Gemfile, ensure it will pick up the pre-release version
gem 'pg', '>= 0.19.0.beta'