chef-cli, inspec, serverspec: Fix Gemfile deprecation warnings

This resolves the following warning in these packages:

[DEPRECATED] This Gemfile does not include an explicit global source. Not using an explicit global source may result in a different lockfile being generated depending on the gems you have installed locally before bundler is run. Instead, define a global source in your Gemfile like this: source "https://rubygems.org".

pkgs/tools/misc/chef-cli/Gemfile

···
-source 'https://rubygems.org' do
-  gem "chef-cli"
-end
+source 'https://rubygems.org'
+gem "chef-cli"

pkgs/tools/misc/inspec/Gemfile

···
-source 'https://rubygems.org' do
-  gem "inspec"
-  gem "inspec-bin"
-end
+source 'https://rubygems.org'
+gem "inspec"
+gem "inspec-bin"

pkgs/tools/misc/serverspec/Gemfile

···
-source 'https://rubygems.org' do
-  gem 'serverspec'
-end
+source 'https://rubygems.org'
+gem 'serverspec'