Add bootstrap form with helper on rails 5.1

[chukitow]

On Rails 5.1 form_tag & form_for were unified in form_with, I'm adding the helper when using Rails >= 5.1

[bootstrap-ruby-bot]

	2 Warnings
⚠️	There are code changes, but no corresponding tests. Please include tests if this PR introduces any modifications in rails-bootstrap-forms’s behavior.
⚠️	Please update CHANGELOG.md with a description of your changes. If this PR is not a user-facing change (e.g. just refactoring), you can disregard this.

Here's an example of a CHANGELOG.md entry (place it immediately under the * Your contribution here! line):

* [#354](https://github.com/bootstrap-ruby/rails-bootstrap-forms/pull/354): Add bootstrap form with helper on rails 5.1 - [@chukitow](https://github.com/chukitow).

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[lcreid]

PR #346 adds support for form_with with extensive testing. [Disclaimer: I submitted that PR.] It now has merge conflicts which I'll resolve once we get a few of the other pending PRs merged into master.

[lcreid]

PR #346 was replaced by PR #369.

[lcreid]

@mattbrictson , could you please review this one, as I have an obvious conflict of interest. :-) If I understand correctly, this PR was submitted against master back when master was still Bootstrap 3.

[mattbrictson]

Closed in favor of #369.