Disallow extra props for matrix

[thejcannon]

This fails with "foo is not a valid property on the matrix configuration. Please check the documentation to get a full list of supported properties."

steps:
  - command: command
    matrix:
      setup:
        test:
          - "A"
          - "B"
      adjustments:
        - with:
            test: "B"
      foo: bar

[angusfretwell]

Closing in favour of #128