v18 - Out with the old!

The update to v18 was largely foundational: removal of bower and update of scripts and CI. It may break someone using Bower to install from git, hence the major bump.

Breaking:

• The script you should include has moved from src/Tween.js to dist/tween.umd.js for global <script> tags. See the install guide.
• Relative start values were removed (they were not documented).
  • Migration:
    • Before: tween.start('+100')
    • After: tween.delay(100).start()