Add shoulda-context and example tests

[ArturT]

Related PR
KnapsackPro/knapsack#37

This PR is using knapsack gem on CI based on the branch: https://github.com/ArturT/knapsack/tree/shoulda-context

How to test if knapsack support for shoulda-context works? Please generate report with

KNAPSACK_GENERATE_REPORT=true bundle exec rake test

The result reports should not contain the wrong path like
/Users/artur/.rvm/gems/ruby-2.3.0/gems/shoulda-context-1.2.1/lib/shoulda/context/context.rb

Correct expected path is test/shoulda_context/calculator_test.rb

Example output for generated report:

{
  "test/minitest_duplicated/meme_unit_test.rb": 0.00022482872009277344,
  "/Users/artur/.rvm/gems/ruby-2.3.0/gems/shoulda-context-1.2.1/lib/shoulda/context/context.rb": 0.0001289844512939453,
  "test/minitest/meme_spec_test.rb": 7.867813110351562e-05,
  "test/minitest_duplicated/meme_spec_test.rb": 0.00011181831359863281,
  "test/controllers/articles_controller_test.rb": 0.4389979839324951,
  "test/models/article_test.rb": 2.6226043701171875e-05,
  "test/controllers/welcome_controller_test.rb": 0.012137889862060547
}

If you want to test knapsack gem from you local repo then update Gemfile and use your own path.

gem 'knapsack', path: ENV['KNAPSACK_REPO_PATH'] || '~/Documents/github/knapsack-pro/knapsack'