Fix backtrace for running the bundle’s own specs.

When running the bundle’s own specs *all* backtrace lines got filtered out, so that RSpec showed the complete backtrace as a fallback.
Fix switch_command_spec.

Broken since 0ecad597.
Remove references to `Spec`
Remove no longer used method `rspec2?`
Remove require statements for RSpec 1

Refactoring in baby steps here, trying to make sure I don’t break anything else.
Simplify conditional
Simplify conditional even more.
Merge branch 'remove-support-for-rspec1' into us-master
Update README (first try)
Remove “lorem”.

How did that sneak in? 😵
Mention legacy version, remove references to RSpec 1.
Recover unintentionally removed text, minimize diff.