Prevent race condition (fixes #134)

[callumlocke]

this fixes it, I no longer get the uncaught ENOENT error I reported in #134

but I notice there's a lot more calls to fs.existsSync throughout the code... might be worth trying to remove them too, for the same reason stated in the issue

[shama]

Interesting, thanks for this! I'll work on updating the other instances of fs.exists and stop using that.