update package.json to 0.5.6

[rbmoore]

Currently the latest tag for the repo (0.5.5) does not match the version in package.json. Can we update to 0.5.6 and get a fresh tag? The npm registry for polymer-context-free-parser could then use a refresh with this version. It is using 0.4.2.

Thanks!

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[rbmoore]

I signed a CLA for rbmoore

[googlebot]

CLAs look good, thanks!

[addyosmani]

This should be easy enough for us to get out. once updated, @azakus do we need to publish the git tagged version of this repo to npm or would master be sufficient?

[rbmoore]

I submitted another pull request that maybe can get in before 0.5.6 is created?