Rakefile updates to support appbundle-updater

InSpec is "appbundled" into the ChefDK which means users can't
simply run `chef gem install inspec` and get an updated InSpec.
`appbundle-updater` exists to combat this and, when run, will
pull the source and install the gem using `rake install`.

In the appbundle-updater config, we'll want to exclude unnecessary
gems from getting installed, such as the "tests" group. Our Rakefile
doesn't work without the "tests" group being installed.

This change makes the Rakefile usuable when the "tests" gems aren't
available.

Signed-off-by: Adam Leff <adam@leff.co>

Rakefile

@@ -4,19 +4,26 @@
 require 'bundler'
 require 'bundler/gem_tasks'
 require 'rake/testtask'
-require 'rubocop/rake_task'
-require_relative 'tasks/docs'
 require_relative 'tasks/maintainers'
 
-# Rubocop
-desc 'Run Rubocop lint checks'
-task :rubocop do
-  RuboCop::RakeTask.new
+# The docs tasks rely on ruby-progressbar. If we can't load it, then don't
+# load the docs tasks. This is necessary to allow this Rakefile to work
+# when the "tests" gem group in the Gemfile has been excluded, such as
+# during an appbundle-updater run.
+begin
+  require 'ruby-progressbar'
+  require_relative 'tasks/docs'
+rescue LoadError
+  puts 'docs tasks are unavailable because the ruby-progressbar gem is not available.'
 end
 
-# lint the project
-desc 'Run robocop linter'
-task lint: [:rubocop]
+# Rubocop
+begin
+  require 'rubocop/rake_task'
+  RuboCop::RakeTask.new(:lint)
+rescue LoadError
+  puts 'rubocop is not available. Install the rubocop gem to run the lint tests.'
+end
 
 # update command output for demo
 desc 'Run inspec commands and save results to www/app/responses'