fix(publish): default lerna publish registry to NPM

[austingreendev]

Description

The most recent Travis builds have been failing due to some authentication issues with the yarn registry. This PR sets the lerna publish --registry command to use the NPM registry exclusively.

I've tested this locally to deploy the packages that were missed in https://travis-ci.org/zendeskgarden/react-components/builds/429674586

Checklist

• 👌 design updates are Garden Designer approved (add the
  designer as a reviewer)
• 💅 view component styling is based on a Garden CSS
  component
• 🌐 Styleguidist demo is up-to-date (yarn start)
• ⬅️ renders as expected with reversed (RTL) direction
• 💂‍♂️ includes new unit and snapshot tests
• 📒 any new files are included in the packages src/index.js export
• 📝 tested in Chrome, Firefox, Safari, Edge, and IE11

[coveralls]

Coverage remained the same at 95.966% when pulling 8d2cf25 on agreen/fix-lerna into b8789bd on master.