Upgrade Chef version immediately instead of waiting until the end of the run

The way the `default` recipe is currently wired, a new version of Chef will not be installed until the very end of the first chef run.

Example:

1. You lock the Chef version to 11.8.2 using the attributes in this cookbook.
2. You perform a standard bootstrap and initial run on a new node using `knife ec2 server create ...`; this installs Chef version 11.10.4.
3. During your run the cookbook downloads the desired version of Chef and triggers a *delayed* notification to install it and kill the current run.
4. **End Result**: The first run always executes using whatever version of Chef was initially installed and only subsequent runs use the version you specified.

I'm not sure why a `ruby_block` resource was used as a notification bus here but, regardless, a notification should trigger immediately. People can then use this as the 1st recipe in their run list and it will ensure that their nodes are locked to a specific version of Chef for ALL runs.