v6.4.0 RC2

To install with npm: npm install foundation-sites@6.4.0-rc2-1
To install with bower: bower install zurb/foundation-sites#v6.4.0-rc2

To try out with the ZURB template: git clone -b v6.4 git@github.com:zurb/foundation-zurb-template.git

Updates from RC1:

1. Update grid classes to use .grid-x and .grid-y rather than .grid and .grid-vertical. Eliminates separate "vertical" classes on cell level, having all inherit from the grid above it. See #10118
2. Fix a bug in Motion UI Javascript. See #10110 (thanks @swey!)
3. Add foundation-prototype.css as a build with prototyping classes enabled to dist.

RC Docs are available here: http://foundation.zurb.com/sites/6.4.0-rc/docs/