Build path sends JSON file path

## What does this MR do?

Fixes an issue with browser caching JSON response on build page.

Steps to see:

- Open build (https://gitlab.com/gitlab-org/gitlab-ce/builds/2030226)
- Close tab
- Re-open tab

You should now see some JSON 😞 

This MR appends `.json` to the end of the build URL so that the browser can't cache it as the same URL.


See merge request !4935