Enforce minimum Node version in CLI

[malept]

Have you read the section in CONTRIBUTING.md about pull requests?

Yes

Summarize your changes:

If Electron Packager's CLI is run with an old version of Node, it will give a SyntaxError which is not intuitive to people what that means. This change (based on a suggestion by @MarshallOfSound in #445 (comment)) gets rid of the SyntaxError, but tells the user exactly why the CLI will not run, before exiting with a non-zero status code. (Example output below.)

Are your changes appropriately documented?

Yes (prior to this PR existing):

• the NEWS file
• the installation instructions
• the engines section of package.json

Do your changes have sufficient test coverage?

No, but it's dependent on Node version, so I felt it wasn't worth doing extra work just to get code coverage for this feature. I tested manually with these instructions on Linux:

1. Install nodeenv
2. Run nodeenv -n 0.12.15 --prebuilt /tmp/n012
3. Run source /tmp/n012/bin/activate
4. Run cd /path/to/electron-packager && ./cli.js

Expected output:

CANNOT RUN WITH NODE 0.12.15
Electron Packager requires Node 4.0 or above.

Does the testsuite pass successfully on your local machine?

Yes