Hold the values for actual/expected when test fails (not references)
Hold the values for actual/expected when test fails (not references)

Summary:related to https://github.com/facebook/jest/issues/429

It can be an expensive operation, but it only occurs when a test fails.

the following test:
```js
  it('keeps the reference', function() {
    var a = [1];
    expect(a).toEqual([1, 2]);
    a.push(2);
  });
```

before:
![screen shot 2016-04-12 at 5 55 40 pm](https://cloud.githubusercontent.com/assets/940133/14479858/ded42e0e-00d7-11e6-8841-0d817ca02d45.png)

after:
![screen shot 2016-04-12 at 5 56 03 pm](https://cloud.githubusercontent.com/assets/940133/14479860/e3d1710a-00d7-11e6-837f-e2f2bdfb27b7.png)
Closes https://github.com/facebook/jest/pull/889

Differential Revision: D3188934

fb-gh-sync-id: 02e8b2e3466d7994a1d3e5e428b80ddd0f455fe5
fbshipit-source-id: 02e8b2e3466d7994a1d3e5e428b80ddd0f455fe5

alvinsj/jest,mpontus/jest,ide/jest,facebook/jest,blainekasten/jest,cpojer/jest,aaron-goshine/jest,cpojer/jest,mpontus/jest,facebook/jest,mpontus/jest,Daniel15/jest,razh/jest,blainekasten/jest,alvinsj/jest,ide/jest,blainekasten/jest,razh/jest,facebook/jest,bookman25/jest,skovhus/jest,bookman25/jest,skovhus/jest,bookman25/jest,ide/jest,Daniel15/jest,aaron-goshine/jest,sampepose/jest,cpojer/jest,Daniel15/jest,aaron-goshine/jest,skawian/jest,skovhus/jest,sampepose/jest,skawian/jest