Change isNode check

[pamelafox]

The isNode function is problematic, because it no longer works. (Discussed in https://github.com/ccampbell/rainbow/issues/213)

Changing it to return false works for our browser use case, and that's what this diff does.

I could alternatively:

• replace it with a more updated check, although there's a range of opinions about what the best check is (https://stackoverflow.com/questions/4224606/how-to-check-whether-a-script-is-running-under-node-js, look at more recent ones).
• remove the code later that tries to require a file if isNode() is true, and then fails. That's what another person did in the issue about it, and that also works.

[pamelafox]

Oops, sorry, I meant to open this on the Khan fork. Obviously the real version needs a better fix than my bandaid. Carry on!