`collection_check_boxes` respects `:index` option for the hidden filed name.

closes #14147
Merge pull request #14736 from nashby/hidden-index

`collection_check_boxes` respects `:index` option for the hidden filed name
Use the index on hidden field
[ci skip] Added link to ruby-lang.org installation.
Merge pull request #14746 from zenati/master

Reference and link to install RVM in the Getting Started Guide. [ci skip]