Fix node deprecation warning on Buffer constructor

[dvallin]

Node 10 currently gives this deprecation warning

(node:12492) [DEP0005] DeprecationWarning: Buffer() is deprecated due to security and usability issues. Please use the Buffer.alloc(), Buffer.allocUnsafe(), or Buffer.from() methods instead.

So this pull request replaces all calls to Buffer::new by Buffer.from

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[dvallin]

signed the CLA

[brendandburns]

LGTM.