tweak upgrade steps
Rename `/upgrade` to `/upgrading-from-rspec-2`.

When RSpec 4 comes out, we'll have a different upgrade page.
Making the link `/upgrading-from-rspec-2` will future proof
this link so that it can stay up at that URL long after
RSpec 4, 5, etc have been released.
Use asciinema videos for the upgrade guide.
Add asciinema videos to home page.
Make the videos faster; the were too slow.
Make home page videos faster.
Merge pull request #27 from rspec/tweak_steps

Tweak upgrade steps