No longer works with Jest 21+

[jamesmoss]

Jest recently made a change that requires all matchers to return a function that returns a message string rather than a string itself.

jestjs/jest#3921

Attempting to use this lib with a version of Jest >= 21 will give you something similar to the error below if any test that uses these matchers fails:

 FAIL  test/reducers/appReducer.test.js
  ● app reducer › should return the initial state

    TypeError: message is not a function

      at getMessage (node_modules/expect/build/index.js:106:16)
      at Object.throwingMatcher [as toEqualImmutable] (node_modules/expect/build/index.js:216:23)
      at Object.<anonymous> (test/reducers/appReducer.test.js:9:3)

[unindented]

Fixed by @njlr.

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v2.0.0 should be compatible with Jest >=21. Let me know how it works for you.