`require': cannot load such file -- ./controllers/

I'm trying to run a test file for a controller, but I'm running into this error:

/home/mariana/.rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/gems/activesupport-4.2.1/lib/active_support/dependencies.rb:274:in `require': cannot load such file -- ./controllers/users_controller (LoadError)

I think rspec can't find the actual files it's supposed to test.

The test file, users_controller_spec.rb:

require 'rails_helper'
require './controllers/users_controller'

describe UsersController do
  describe '#create' do
      ...
  end
end

The header/preamble of rails_helper.rb, that deals with file paths (I think):

ENV['RAILS_ENV'] ||= 'test'
require File.expand_path('../../config/environment', __FILE__)
abort("The Rails environment is running in production mode!") if Rails.env.production?
require 'spec_helper'
require 'rspec/rails'

The section of the Gemfile that deals with rspec and development/test-specific stuff:

group :development, :test do
  gem 'spring'
  gem 'rspec', '~>3.5'
  gem 'rspec-rails', '~>3.5' 
end

The project tree:

   ->project
      ->app
        ->controllers
          ->users_controller.rb
      ->spec
        ->controllers
          ->users_controller_spec.rb

I've tried messing around with the require File.expand_path(...) line on rails_helper.rb. I also tried adding the rspec gem to the Gemfile alongside rspec-rails (the problem also existed when it was just rspec-rails).

Any suggestions?