Semver4

1. Remove the engines and nodeVersionRequired business, since `0.10.31-pre` will no longer satisfy semver4's range semantics, and Node v0.6 is pretty well dead anyhow.
2. Update `sha` module to pick up a new readable-stream, again to avoid a -prerelease issue.
3. Update semver to 4.0.0 (and doc/misc/semver.md along with it)