Adding support for IE 11
Update dependencies, supported/tested ruby versions and test suite

Converted specs to RSpec 3.0 syntax, the conversion was done by
Transpec 2.2.0 with the following command:
    transpec

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

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

* 12 conversions
    from: it { should ... }
      to: it { is_expected.to ... }

* 12 conversions
    from: its([:key]) { }
      to: describe '[:key]' do subject { super()[:key] }; it { } end

* 4 conversions
    from: be_false
      to: be_falsey

* 2 conversions
    from: be_true
      to: be_truthy

See also: https://github.com/yujinakayama/transpec#supported-conversions
Add support for Postbox (Closes #2)