v2.1.0

Updated to Bootstrap v4.0.0-beta.2 and added Yarn install option.
Also, from Bootstrap's beta 2 ship list:

• Restored grid .offset- modifiers in #23445. Originally our intent was to supplement this with our margin utilities, but we underestimated the appeal of fixed number column offsets.
• Dropped support for Bower as they've deprecated the package manager. See #23568.
• 24202 - Add .order-last and responsive variations as a new grid modifier class.