Remove rspec-spinner.

It was never updated to work with RSpec 2. It's `example_failed`
implementation takes 3 arguments:

https://github.com/nofxx/rspec_spinner/blob/eeea8961197e07ad46f71442fc0dd79c1ea26ed3/lib/rspec_spinner/base.rb#L52

…which aligns with RSpec 1:

https://github.com/dchelimsky/rspec/blob/v1.3.2/lib/spec/runner/formatter/html_formatter.rb#L67

…but never worked on RSpec 2, which only takes 1 argument:

https://github.com/rspec/rspec-core/blob/4039e7329aa10f9f653227d37a68fc232aaf5cb7/lib/rspec/core/formatters/html_formatter.rb#L72