Remove 'socket' example PostgreSQL parameter

The ActiveRecord PostgreSQL adapter does not have a 'socket' option;
see:

https://github.com/rails/rails/blob/4-1-2/activerecord/lib/active_record/connection_adapters/postgresql_adapter.rb#L20

config/database.yml.postgresql

@@ -10,7 +10,6 @@ production:
   # password:
   # host: localhost
   # port: 5432 
-  # socket: /tmp/postgresql.sock
 
 #
 # Development specific
@@ -22,7 +21,6 @@ development:
   pool: 5
   username: postgres
   password:
-  # socket: /tmp/postgresql.sock
 
 #
 # Staging specific
@@ -34,7 +32,6 @@ staging:
   pool: 5
   username: postgres
   password:
-  # socket: /tmp/postgresql.sock
 
 # Warning: The database defined as "test" will be erased and
 # re-generated from your development database when you run "rake".
@@ -46,4 +43,3 @@ test: &test
   pool: 5
   username: postgres
   password:
-  # socket: /tmp/postgresql.sock