Distribute bin/suspenders, not bin/rspec, in gem

While installing suspenders:

    % gem install suspenders
   suspenders's executable "rspec" conflicts with rspec-core
   Overwrite the executable? [yN]

This is because we added an `rspec` binstub to `bin/` and automatically
distribute all files in `bin/` in the `.gemspec.`

Resolves https://github.com/thoughtbot/suspenders/issues/288