Update to Ruby 2.1.2
Convert specs to RSpec 2.14.5 syntax with Transpec

This conversion is done by Transpec 2.3.7 with the following command:
    transpec spec/

* 9 conversions
    from: obj.should_receive(:message)
      to: expect(obj).to receive(:message)

* 5 conversions
    from: == expected
      to: eq(expected)

* 5 conversions
    from: obj.should
      to: expect(obj).to

* 3 conversions
    from: obj.stub(:message)
      to: allow(obj).to receive(:message)

* 1 conversion
    from: lambda { }.should
      to: expect { }.to

For more details: https://github.com/yujinakayama/transpec#supported-conversions
Remove highline dependency
Rescue Zlib::BufError and consider it as invalid png