Update readme

I discovered that Homebrew installs a 9.3 version of Postgresql.  When I attempted to start my Postgresql server in my dev directory, it informed me that the app needed a 9.2 version of Postgresql.  I've added a note to the install instructions to make this clear.
Removing the three back ticks breaks the code block. See: https://github.com/nellshamrell/supermarket/blob/update-readme/README.md