Upgrade node-semver

node-semver 2.3.0 adds premajor, preminor, and prepatch, and changes the behavior of prerelease to something more consistent with semver standards. Most notably, when using prerelease when no prerelease number exists, it will also bump the patch number, because just changing the prerelease to 0 would be going *backwards*. This is the same functionality as offered by prepatch, except prepatch will *always* bump the patch and set prerelease to 0. This is the same behavior found in preminor and premajor, but bumps the major and minor version respectively.