Added support parallel_tests (#2740)

- Added new gem the `parallel_tests`.
- Updated .travis.yml.
  - Improved scripts and environment variable.

Gemfile

@@ -77,6 +77,7 @@ group :test do
   gem 'rspec-sidekiq'
   gem 'simplecov', require: false
   gem 'webmock'
+  gem 'parallel_tests'
 end
 
 group :development do