Added support parallel_tests (#2740)

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

.travis.yml

@@ -14,6 +14,9 @@ env:
     - LOCAL_HTTPS=true
     - RAILS_ENV=test
     - CXX=g++-4.8
+    - NOKOGIRI_USE_SYSTEM_LIBRARIES=true
+    - PARALLEL_TEST_PROCESSORS=2
+
 addons:
   postgresql: 9.4
   apt:
@@ -31,7 +34,7 @@ rvm:
 services:
   - redis-server
 
-bundler_args: --without development production --retry=3 --jobs=3
+bundler_args: --without development production --retry=3 --jobs=16
 
 install:
   - nvm install
@@ -40,10 +43,10 @@ install:
   - yarn install
 
 before_script:
-  - bundle exec rails db:create db:schema:load
+  - bundle exec rake parallel:create parallel:load_schema parallel:prepare
   - bundle exec rails assets:precompile
 
 script:
-  - bundle exec rspec
+  - bundle exec parallel_test spec/ --group-by filesize --type rspec
   - npm test
   - i18n-tasks unused