herokuにrailsアプリをpushしようとするとDetected sqlite3 gem which is not supported on Heroku:エラーが出る

herokuにrails 5アプリをgit push heroku masterしようとすると、エラーが出てしまいます。

$ git push heroku master
Counting objects: 210, done.
Compressing objects: 100% (193/193), done.
Writing objects: 100% (210/210), 49.54 KiB | 0 bytes/s, done.
Total 210 (delta 52), reused 0 (delta 0)
remote: Compressing source files... done.
remote: Building source:
remote:
remote:  !     Warning: Multiple default buildpacks reported the ability to handle this app. The first buildpack in the list below will be used.
remote:                         Detected buildpacks: Ruby,Node.js
remote:                         See https://devcenter.heroku.com/articles/buildpacks#buildpack-detect-order
remote: -----> Ruby app detected
remote: -----> Compiling Ruby/Rails
remote: -----> Using Ruby version: ruby-2.4.2
remote: -----> Installing dependencies using bundler 1.15.2

...

remote:        An error occurred while installing sqlite3 (1.3.13), and Bundler cannot
remote:        continue.
remote:        Make sure that `gem install sqlite3 -v '1.3.13'` succeeds before bundling.
remote:
remote:        In Gemfile:
remote:          sqlite3
remote:
remote:  !
remote:  !     Failed to install gems via Bundler.
remote:  !     Detected sqlite3 gem which is not supported on Heroku:
remote:  !     https://devcenter.heroku.com/articles/sqlite3
remote:  !
remote:  !     Push rejected, failed to compile Ruby app.
remote:
remote:  !     Push failed
remote: Verifying deploy...
remote:
remote: !       Push rejected to quiet-sands-44976.
remote:
To https://git.heroku.com/quiet-sands-44976.git
 ! [remote rejected] master -> master (pre-receive hook declined)
error: failed to push some refs to 'https://git.heroku.com/quiet-sands-44976.git'

やったこと

herokuのセットアップ

heroku login
gemfileに`gem 'pg'`を追加
sudo apt-get install libpq-dev
bundle install

gemfileからsqlite3の削除。その後、bundle install
依存関係の確認。gemfile.lockにsqlite3が含まれていないか確認。含まれていませんでした。
database.ymlの編集。production環境はもちろんのこと、他の環境からもsqliteを除外した。

config\database.yml

# SQLite version 3.x
#
# default: &default
#   adapter: sqlite3
#   pool: 5
#   timeout: 5000
#   encoding: utf8
#
# development:
#   <<: *default
#   database: db/development.sqlite3
#
test:
  adapter: mysql2
  encoding: utf8
  database: aptnote_test
  pool: 5
  username: user
  password: password
  host: localhost

production:
  adapter: postgresql
  database: my_database_production
  pool: 5
  timeout: 5000

development:
  adapter: mysql2
  encoding: utf8
  database: aptnote_development
  pool: 5
  username: user
  password: password
  host: localhost
  # socket: /tmp/mysql.sock