Postgres & Rails – FATAL: password authentication failed for user

Newbie to Postgres, basic skills with Rails, so bear with me.

I will explain the process of how I got to where I am, in case I did anything wrong along the way.

What I did.

I created a Rails 5 project (using the actual Git repo) and, once it was created successfully, I found the default gem sqlite3 in the Gemfile.

Now, I want to work with Postgres, so I turned sqlite3 into pg, and ran bundle install. Worked perfectly, Rails didn't complain.

Then, I installed Postgres on my computer (using the latest OS X) and ran createdb testDB. It's an empty database.

Now, I have to configure the database.yml file. After going through a dozen of links and resources, I changed the file's contents to the following:

default: &default
  adapter: postgresql
  database: testDB
  username: postgres
  password: password
  host: localhost
  port: 9057


development:
  <<: *default

test:
  <<: *default

production:
  <<: *default
  database: someOtherDB

I am certain that the port number is correct, because I set it manually, and I can check that it's running using the command netstat -na. I get the following two lines:

tcp6       0      0  *.9057                 *.*                    LISTEN     
tcp4       0      0  *.9057                 *.*                    LISTEN    

Now, in order to change the password for postgres to ensure it's actually the cleverly secure string password, I ran the following commands:

$ sudo -u postgres psql template1

Opens up the psql prompt, then I run the command:

psql (9.3.4)
Type "help" for help.

template1=# ALTER USER postgres PASSWORD 'password';

Which returned:

ALTER ROLE

Yay! Password's changed! Now I can open up my first ever working Rails app with a Postgres backend!

What I got.

I run bin/rails server, and I get the following error:

Some more info that may be helpful.

When I run the command psql -l, I get the following:

                                        List of databases
   Name    |    Owner     | Encoding |   Collate   |    Ctype    |       Access privileges       
-----------+--------------+----------+-------------+-------------+-------------------------------
 postgres  | myusername | UTF8     | en_GB.UTF-8 | en_GB.UTF-8 | 
 template0 | myusername | UTF8     | en_GB.UTF-8 | en_GB.UTF-8 | =c/myusername              +
           |              |          |             |             | myusername=CTc/myusername
 template1 | myusername | UTF8     | en_GB.UTF-8 | en_GB.UTF-8 | =c/myusername              +
           |              |          |             |             | myusername=CTc/myusername
 test      | myusername | UTF8     | en_GB.UTF-8 | en_GB.UTF-8 | 
 testDB    | myusername | UTF8     | en_GB.UTF-8 | en_GB.UTF-8 | 
(5 rows)

If I run psql X (where X is any of the databases listed above) followed by running the command \du, I get the following.

                               List of roles
  Role name   |                   Attributes                   | Member of 
--------------+------------------------------------------------+-----------
 myusername   | Superuser, Create role, Create DB, Replication | {}
 postgres     | Superuser                                      | {}

I followed instructions on this link and a few others to no avail.

What am I doing wrong?

Thank you so much in advance.

EDIT: Here's the content of log/development.log.