package.json: Require a specific version for 'source-map'.

This will ensure reproducible builds for any checked-out commit of traceur.

BUG=https://github.com/google/traceur-compiler/issues/243
TEST=None

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I'm using source-map 0.1.16 as the baseline because that's the version that seems to be most common in the commits of `bin/traceur.js` that followed 2070a8aeb6406db0 Wrap the newest version of 'source-map'.

Unfortunately, you can't reproduce the other builds without some (possibly computer-assisted) trial and error.

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In terms of the motivation for this patch, not much to say at this point beyond my comments in #243.