Proper lint for rubocop-rspec

[ka8725]

The default configuration of Rubocop RSpec fails to detect constructions its_block/its_call/its_map defined by Saharaspec.

This PR fixes the issue.

Consider the following example:

# cat spec/example_spec.rb
describe 'Saharaspec' do
  context 'text' do
    subject { 'x' }

    its_block { is_expected.to eq 'x' }
  end
end

As default rubocop command fails as this:

spec/example_spec.rb:2:3: C: RSpec/EmptyExampleGroup: Empty example group detected.
  context 'text' do
  ^^^^^^^^^^^^^^

Install the fix locally updating Gemfile as follows:

gem 'saharspec', require: false, path: '<path_to_local_dir>/saharspec'

Add these lines to .rubocop.yml of you project:

inherit_gem:
  saharspec: config/rubocop-rspec.yml

And now Rubocop has no complains.

This is a follow up on this issue rubocop/rubocop-rspec#975