only install once if called multiple times in same vm

I have a problem using this module in my unit tests, because I don't
have a central place to put the

    require('source-map-support').install();

statement.  So I end up having to put it in every unit test, and once
I've written enough tests, I start getting an annoying error about using
up too much memory.

This change makes is so if you call install() multiple times in the same
vm, it only actually installs once.