Mounts a separate sprockets environment for each tenant's assets

* Each environment is mounted at /tenants/#{tenant_name}/assets
Ensures that test assets are available before test app boots
Moves test_app_fixtures out of support directory
Implements fall back to app assets
Deletes product data before running empty store examples
Makes sprockets helpers tenant aware
Ignores .ruby-version
Moves open3 to where it's used
Adds rake tasks for precompiling tenant assets
Ensures that tenant assets exists before trying to reference them
Makes validator on Spree::Product#permalink tenant aware
Adds failing specs for requiring files from css manifests

I need to put this work on hold for a little bit, but someone else
may be able to pick up where I've left off.

Problems to solve:
* It looks like vendor directories from the spree_core gem are not
  getting added to the tenant asset path. I suspect an issue with
  `Spree::SpreeLandlord::Assets::Bootstrap#run on lines 19-22
* Need to figure out a reliable way to verify that scss has been
  compiled correctly. The formatting does not seem to be preserved
  which will make those specs brittle. It may be useful to use a
  css parser to compare the content in a way that ignores formatting
  and comments.
Merge branch 'master' into topic/tenant-assets
Corrects require from included gem example

spree_core no longer includes html5reset. It's been replaced with
normalize
Sanitizes css content before making a comparison

Sass::CSS is used to convert the structure of the css and then a
regular expression is used to filter out comments and new lines.
Only load Sass support when Sass is loaded
Ensures that the correct asset manifest is used
Adds handling assets for each tenant

* Assets are stored at
  /app/tenants/<tenant_name>/assets/[stylesheets, etc...]
* To precomplie tenant assets run `rake tenants:assets:precompile`
  * This will create an asset manifest at
    /public/tenants/<tenant_name>/assets/manifest.yml
* All asset path helpers detect the current tenant and direct will
  return the path to the assets for that tenant. If there are no assets
  for that tenant, than the app's assets are used.
* If a tenant does not provide a requested asset, it is searched for in
  the app assets directories and then the other assets path locations
  such as /vendor and any assets that are provided by gems
* You'll still need to precompile app assets for appropriate fallback
  to work correctly.