Require spec/spec_helper by default.

This means that spec files (e.g. user_spec.rb) don't need `require
'spec_helper'` at the top.

We removed this in 66a26d39593da6ad9ea5ff129126e1e635a5ab38 because the options
in `.rspec` were user-specific (e.g. `--color`). We are re-adding it because
requiring `spec_helper` is specific to the project.


Previous discussion: https://github.com/thoughtbot/suspenders/pull/214