Github Pages deployment fix

[evanmwillhite]

Fixes github pages deployment script (gulp ghpages-deploy) by switching from gulp-gh-pages to gh-pages npm package. This PR merely ignores the build directory created by the script - the majority of the change is in fourkitchens/emulsify-gulp#74

To test:

• Fork Emulsify and clone the new fork down locally
• Open package.json and change fourkitchens/emulsify-gulp#v2.0.0 to fourkitchens/emulsify-gulp#pages-fix
• yarn install
• yarn start
• gulp ghpages-deploy
• Verify the gh-pages branch was created and push to your repo and that in your repo's settings, you can navigate to the Github Pages url + /pattern-lab/public and view the Pattern Lab instance.

[ccjjmartin]