Merge pull request #125 from bitzesty/reduce-crawler-load

Reduce crawler load
Use `X-Robots-Tag` header in controllers that inherit from `ApplicationController`

@matthewford's comment on PT:
> @macool The robots meta works in the search, but also needs to be added in the other controllers for headings, sections, commodities too.

- Add `X-Robots-Tag` header on historical pages
- Added a spec for `SectionsController` related to `X-Robots-Tag`

[pivotal story](https://www.pivotaltracker.com/n/projects/488191/stories/68707106)