Bootstrap V4 Alpha checkbox fixes

[trammel]

This is based on #299 and adds support for the changed class names for checkboxes in Bootstrap v4 alpha.

It also relates to #268 but, I don't see how it'll be possible to easily support Bootstrap v3 & v4 in the same version of the gem.

[bgentry]

Perhaps it would make sense to merge this into a v4-alpha working branch on this repo? Then others could continue submitting fixes for v4 up until its non-beta release.

Edit: I see there's a bootstrap-v4 branch but nothing has been pushed to it. Maybe just change this PR to merge there, and then merge if it's ok?

[desheikh]

@trammel @antpaw looks like we ended up duplicating this work. It doesn't look like there's anything new against the bootstrap-v4 branch, Can we close this?

[GBH]

This PR should be closed as it doesn't work with Bootstrap4 beta3. has_danger is not a thing and form group doesn't have class to indicate errors anymore.

[mattbrictson]

@trammel I'm closing this PR for now, but I'd appreciate your help if you are still interested. Refer to #361 to see where are currently are on the Bootstrap v4 roadmap. Thanks!