How to run Rubocop only on the changed files in a pull request?

I have created spec/lint/rubocop_spec.rb which runs Rubocop style checker on the files changed between current branch and master. This works when I test locally but not when the test run on the build server Circle.ci. I suspect it is because only the branch in question is downloaded, so it does not find any differences between master. Is there a better way than git co master && git pull origin master? Can I query the Github API perhaps to get the files changed listed?

require 'spec_helper'

describe 'Check that the files we have changed have correct syntax' do
  before do
    current_sha = `git rev-parse --verify HEAD`.strip!
    files = `git diff master #{current_sha} --name-only | grep .rb`
    files.tr!("\n", ' ')
    @report = 'nada'
    if files.present?
      puts "Changed files: #{files}"

      @report = `rubocop #{files}`
      puts "Report: #{@report}"
    end
  end

  it { @report.match('Offenses').should_not be true }
end

I fixed it by querying api.github.com. This will run rubocop on all files that has been changed between current_sha and the master branch.

require 'spec_helper'

describe 'Check that the files we have changed have correct syntax' do
  before do
    current_sha = `git rev-parse --verify HEAD`.strip!
    token = 'YOUR GITHUB TOKEN'
    url = 'https://api.github.com/repos/orwapp/orwapp/compare/' \
          "master...#{current_sha}?access_token=#{token}"
    files = `curl -i #{url} | grep filename | cut -f2 -d: | grep \.rb | tr '"', '\ '`
    files.tr!("\n", ' ')
    @report = 'nada'
    if files.present?
      puts "Changed files: #{files}"

      @report = `rubocop #{files}`
      puts "Report: #{@report}"
    end
  end

  it { expect(@report.match('Offenses')).to be_falsey }
end