replace custom release scripts in favour of np

[zinserjan]

np is much more powerful than the current release script and avoids some release errors.

• Ensures you are publishing from the master branch
• Ensures the working directory is clean and that there are no unpulled changes
• Reinstalls dependencies to ensure your project works with the latest dependency tree
• Runs the tests
• Bumps the version in package.json and npm-shrinkwrap.json (if present) and creates a git tag
• Prevents accidental publishing of pre-release versions under the latest dist-tag
• Publishes the new version to npm, optionally under a dist-tag
• Pushes commits and tags to GitHub

see here