Add Gemfile predictor strategy

If the Gemfile (or Gemfile.lock) has changed, all tests neeed to be run
to ensure correctness.

lib/crystalball.rb

@@ -7,6 +7,7 @@
 require 'crystalball/rspec/runner'
 require 'crystalball/prediction'
 require 'crystalball/predictor'
+require 'crystalball/predictor/gemfile'
 require 'crystalball/predictor/modified_execution_paths'
 require 'crystalball/predictor/modified_specs'
 require 'crystalball/predictor/modified_support_specs'

lib/crystalball/predictor/gemfile.rb

@@ -0,0 +1,26 @@
+require 'crystalball/predictor/strategy'
+
+module Crystalball
+  class Predictor
+    # Used with `predictor.use Crystalball::Predictor::Gemfile.new`. Return every file if the Gemfile or Gemfile.lock file are changed
+    class Gemfile
+      include Strategy
+
+      # @param [Crystalball::SourceDiff] diff - the diff from which to predict
+      #   which specs should run
+      # @param [Crystalball::ExampleGroupMap] map - the map with the relations of
+      #   examples and used files
+      # @return [Array<String>] the spec paths associated with the changes
+      def call(diff, map)
+        super do
+          return [] unless diff.detect{ |file_diff| ['Gemfile', 'Gemfile.lock'].include?(file_diff.path) }
+          map.example_groups.keys.compact
+        end
+      end
+
+      private
+
+      attr_reader :spec_pattern
+    end
+  end
+end

spec/predictor/gemfile_spec.rb

@@ -0,0 +1,31 @@
+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe Crystalball::Predictor::Gemfile do
+  subject(:predictor) { described_class.new }
+  let(:repository) { Git::Base.new }
+  let(:path1) { 'Gemfile' }
+  let(:file_diff1) { Crystalball::SourceDiff::FileDiff.new(Git::Diff::DiffFile.new(repository, path: path1)) }
+  let(:diff) { [file_diff1] }
+  let(:map) { instance_double('Crystalball::MapGenerator::ExecutionMap', example_groups: example_groups) }
+  let(:example_groups) { {'spec_file' => [], 'spec_file_2' => [] } }
+
+  describe '#call' do
+    subject { predictor.call(diff, map) }
+
+    it { is_expected.to eq(['./spec_file', './spec_file_2']) }
+
+    context 'when gemfile.lock is included in changed files' do
+      let(:path1) { 'Gemfile.lock' }
+
+      it { is_expected.to eq(['./spec_file', './spec_file_2']) }
+    end
+
+    context 'when neither Gemfile or Gemfile.lock are changed' do
+      let(:path1) { 'version.rb' }
+
+      it { is_expected.to eq([]) }
+    end
+  end
+end