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.