To migrate postgres, I think this is a good tutorial about installing and configuring postgres in mac. With the ruby gem installing postgres 9.4.1. I had tweak some commands discussed in the tutorial. The switch from mysql to postgres was easy, but one thing that I was unable to get going was to use spring with my migration. So I had to comment the gem for spring and run the db:migrate and come back uncomment the spring gem.
How to Setup PostgreSQL for Rails Development on OS X - YouTube: "https://youtu.be/pf5jPUJAeU4"