Fix jest-haste-map on vagrant.

[cpojer]

This is a similar fix to what @spicyj did in sophiebits@cff1410 a long time ago. I wasn't aware of why we had this but it seems obvious to me that we shouldn't create processes if there are less than one worker available. Not really sure how that could happen but it seems that on vagrant/docker installs the number of cores is 0.

See #980.

[cpojer]

@facebook-github-bot import

[ghost]

Thanks for importing. If you are an FB employee go to Phabricator to review.

[zpao]

Any chance this is the same fix needed to make coverage and runInBand work together?

[cpojer]

Probably not, looks like a different bug unfortunately :(

[cpojer]

We really need to start using flow + add more integration tests.

[janaz]

I think the proper fix should be around here (assuming that options.maxWorkers should represent the number of cores)
https://github.com/facebook/jest/blob/master/packages/jest-cli/src/jest.js#L41-L42

In the highlighted lines the options.maxWorkers is set to 0 for when cpus equals 1

[StephanBijzitter]

I can confirm this PR fixes my issue.

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