Specify multi_json dependency < 1.9

https://travis-ci.org/yujinakayama/transpec/builds/20377379
Convert specs to RSpec 2.99.0.beta2 syntax with Transpec

This conversion is done by Transpec 1.10.4 with the following command:
    transpec --force --convert-stub-with-hash

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

* 61 conversions
    from: be_false
      to: be_falsey

* 37 conversions
    from: be_true
      to: be_truthy

* 11 conversions
    from: lambda { }.should
      to: expect { }.to

* 4 conversions
    from: =~ /pattern/
      to: match(/pattern/)

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

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

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