Update Page Model for Rails 5

[davidstump]

belongs_to is now explicitly required in the Rails 5 beta. In this situation, an optional parent_id is needed to set a root page with no parent.

Not sure if you want to pull in changes for the beta version of Rails 5 - but I figured I would send it your way and let you decide.

Cheers.

[scytherswings]

If this PR is updated, what else is necessary for rails 5 support? I see this fork: https://github.com/NarasimhaReddyY/documentation from @narasimhareddy has a few more changes for 5.1 which looks promising. Might you have any time to look at this @adamcooke ?

[fwolfst]

I believe that I got it running by adding the "[5.2]" to the migrations (e.g. ... < ActiveRecord::Migration[5.2]). Didnt do much tests, though.