Bump version in npm-shrinkwrap.json if available

A good practice for npm-shrinkwrap is to visually confirm the changes in the file before committing and generating a release via `grunt release`. But now the package.json version and npm-shrinkwrap.json versions are out of sync, since only the former is bumped.

Someone else suggested *generating* the shrinkwrap as part of grunt-release (#39), but I think simply incrementing would be a better addition.

Thoughts?