Add formula for node@7

[paulelliott]

• Have you followed the guidelines for contributing?
• Have you checked that there aren't other open pull requests for the same formula update/change?
• Have you built your formula locally with brew install --build-from-source <formula>, where <formula> is the name of the formula you're submitting?
• Does your build pass brew audit --strict <formula> (after doing brew install <formula>)?

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This is a copy of the most recent node 7.x version from the existing node formula. It is now tracking the 8.x line and many people still need to pin down to node 7.

[ilovezfs]

Please see #14118.