set up gem for github_api

plugins/workflow_report/Gemfile

+source 'https://rubygems.org'
+gem 'addressable', '~> 2.5', '>= 2.5.2'
+gem 'descendants_tracker', '~> 0.0.4'
+gem 'faraday', '~> 0.17.6'
+gem 'hashie', '~> 3.5', '>= 3.5.7'
+gem 'oauth2', '~> 1.2'
 gem 'github_api', '~> 0.19.0'