Fix page tree cache refresh on sub page changes

[aiomaster]

Hi,
it took me some hours to find and fix a problem that came in with admin page index collection caching (see a1b6e8a).
Problem was, that the page tree showed old pages if we changed a page title on a sub child of one of the children of a page.
The collection caching just checked the children of a page, but not if a child (or child of a child ...) was changed. So we ended with a wrong page tree.
I wrote a test and fixed the problem by including the descendants of a page into the caching decision.
Cheers!

[aiomaster]

I really don't know why the travis tests with mysql fail. I can not reproduce it on my local machine. Strange!

[aiomaster]

Funny, it was a caching problem :)
added a Rails.cache.clear seems to fix it.

[bricesanchez]

@aiomaster thanks you so much, i did not test this feature on a sub child!