v1.6.0 Update to use Propshaft

This allows the gem to work with Rails applications with Propshaft

• Removes bundled Bootstrap CSS
• Links Bootstrap CSS from CDN
• Updates styling
• Removes sassc-rails gem
• Adds propshaft gem
• Removed vendor folder from gem

What's Changed

• Migrate to Propshaft by @JavaKoala in #7

Full Changelog: v1.5.0...v1.6.0