Travisのビルド時に下記のようなエラーが発生してしまいます。
原因は何でしょうか?

実行コマンドはruby test/sample_test.rbです。
ローカルでは問題なく実行完了します。(テストが通ります)

ビルドログに出力されるエラー箇所

/home/travis/.rvm/rubies/ruby-2.0.0-p247/lib/ruby/site_ruby/2.0.0/rubygems/core_ext/kernel_require.rb:45:in `require': cannot load such file -- sinatra (LoadError)
    from /home/travis/.rvm/rubies/ruby-2.0.0-p247/lib/ruby/site_ruby/2.0.0/rubygems/core_ext/kernel_require.rb:45:in `require'
    from /home/travis/build/handshakejp/api/app.rb:2:in `<top (required)>'
    from test/sample_test.rb:3:in `require_relative'
    from test/sample_test.rb:3:in `<main>'

test/sample_test.rb

ENV['RACK_ENV'] = 'test'

require_relative '../app.rb'
require 'test/unit'
require 'rack/test'

class SampleTest < Test::Unit::TestCase
  include Rack::Test::Methods

  def app
    Sinatra::Application
  end

  def test_it_says_hello_world
    get '/'
    assert last_response.ok?
    assert last_response.body.include?('hello')
  end
end

app.rb

require 'sinatra'

get '/' do
  @text = 'hello'
  erb :index
end

Gemfile

source 'https://rubygems.org'
gem 'sinatra'
gem 'rake'
#gem 'parse-ruby-client'
#gem 'rubocop', require: false
#gem 'rack-test', require: 'rack/test'