Use generated binstubs in guides examples.

Discussed in https://groups.google.com/forum/#!topic/rubyonrails-core/yY5le0_VsDQ

I updated all examples (in grey box with cogwheel) I've found and where it make sense to use binstub.

I skipped commands lists. For example look at start of http://guides.rubyonrails.org/command_line.html.