Add iPhone 6 Plus, 6, 5s and 4s variants to devices file (with updated user agent strings for iOS 8.1.1). Set iPhone 6 to default. Added tests for 6 and 6 plus.
Convert specs to RSpec 3.0.3 syntax with Transpec

This conversion is done by Transpec 2.3.8 with the following command:
    transpec

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

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

* 6 conversions
    from: obj.should_not
      to: expect(obj).not_to

For more details: https://github.com/yujinakayama/transpec#supported-conversions