User murmur instead of md5

[stefanmb]

MD5 is disallowed in FIPS mode, use a pure JS
version of murmur instead.

Resolves #6
Based on npm/fs-write-stream-atomic#6

[stefanmb]

@othiym23 @iarna Does this look okay? Thanks again for your help.

[othiym23]

As you noticed, I did the same thing in 22dd875, mostly because I thought this was the PR against fs-write-stream-atomic. My apologies! Thanks for putting this together, and I hope the fact that [email protected] works with FIPS makes up for me inadvertently bigfooting you here. Thanks for getting the ball rolling with this!