Merge branch 'build-reopen-json' into 'master'

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

app/views/projects/builds/show.html.haml

@@ -67,4 +67,4 @@
 = render "sidebar"
 
 :javascript
-  new CiBuild("#{namespace_project_build_url(@project.namespace, @project, @build)}", "#{@build.status}", "#{trace_with_state[:state]}")
+  new CiBuild("#{namespace_project_build_url(@project.namespace, @project, @build, :json)}", "#{@build.status}", "#{trace_with_state[:state]}")