Specify null false for timestamps
Specify null false for timestamps

Fix Rails 5 deprecation warning for not explicitly setting the null option
for timestamps. Also moved other timestamps to use timestamps method

This deprecation warning was introduced in https://github.com/rails/rails/pull/16481

fabien-michel/closure_tree,mceachen/closure_tree,mceachen/closure_tree,mceachen/closure_tree,seuros/closure_tree,procore/closure_tree,fabien-michel/closure_tree,froderik/closure_tree,froderik/closure_tree,procore/closure_tree,seuros/closure_tree,froderik/closure_tree,fabien-michel/closure_tree,procore/closure_tree,seuros/closure_tree