Fix gem conflicts in ruby environments that load rake 11

[danielsdeleo]

#1. Allow rake 11.x.

Something in our Rakefile is using a deprecated API, but it doesn't seem to be breaking anything:

bundle exec rake -T
[DEPRECATION] `last_comment` is deprecated.  Please use `last_description` instead.
[DEPRECATION] `last_comment` is deprecated.  Please use `last_description` instead.
rake build               # Build ohai-8.12.0.gem into the pkg directory
rake changelog           # Generate a Change log from GitHub
rake clean               # Remove any temporary products
rake clobber             # Remove any generated files
rake install             # Build and install ohai-8.12.0.gem into system gems
rake install:local       # Build and install ohai-8.12.0.gem into system gems without network access
rake release[remote]     # Create tag v8.12.0 and build and push ohai-8.12.0.gem to Rubygems
rake spec                # Run RSpec code examples
rake style               # Run RuboCop
rake style:auto_correct  # Auto-correct RuboCop offenses

#2. Make rake a dev dependency.

Running git grep -i rake -- lib gives zero results, so we probably just overlooked making rake a dev dep in the past.

[adamedx]

👍

[mwrock]

👍

[grosser]

❤️ release coming ? :)

[danielsdeleo]

@grosser it's on rubygems.org now.