Enforce minimum Node.js version requirement using 'engines' attribute package.json

[compumike08]

Since the cross-env can only support Node.js version 4.0 or greater as of cross-env version 3.0.0, I added an engines attribute to package.json which will enforce the presence of Node.js version 4.0 or greater. If someone tries to run/build cross-env with an older version of Node.js, the engines attribute will cause an error to be thrown as soon as they try to run npm install (or otherwise try to run/build the project).

In addition, I also added a Prerequisites section to README.md which states that Node.js version 4.0 or greater is required.

[codecov-io]

Current coverage is 100% (diff: 100%)

Merging #48 into master will not change coverage

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[kentcdodds]

Thank you!

[compumike08]

@kentcdodds: You're welcome.

[compumike08]

@kentcdodds: Out of curiosity, do you have a regular release schedule for creating new releases and publishing them to the npm registry, or do you create releases as needed?

[kentcdodds]

It's automated. This has already been released! https://github.com/kentcdodds/cross-env/releases

[compumike08]

@kentcdodds: Oh, cool! What do you use to automate the release process?

[kentcdodds]

See this: https://kcd.im/write-oss and/or: https://kcd.im/fem-oss