Remove start_as_center? specs for Dimension

The #start_as_center? method has become a private implementation
detail. Since the specs already check the Dimension values for
correctness in the normal (starting as left) and starting at center
cases, these checks are not needed (plus they raise errors in RSpec
3).
Don't redefine ONE_PIXEL
Convert specs to RSpec 2.99.0.rc1 syntax with Transpec

Note that this commit keeps 3 "old syntaxes":

- its: The conversion that Transpec does is ugly
- stub: Converted specs failed after running this conversion. Needs
  investigation
- should_receive: Converted specs failed after running this conversion. Needs
  investigation

. .. . ... .

This conversion is done by Transpec 2.1.0 with the following command:
    transpec --rspec-command "rake spec" --keep its,stub,should_receive

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

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

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

* 34 conversions
    from: it { should_not ... }
      to: it { is_expected.not_to ... }

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

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

* 3 conversions
    from: expect(collection).to have(n).items
      to: expect(collection.size).to eq(n)

See also: https://github.com/yujinakayama/transpec#supported-conversions