Introduced submodule for Jekyll plugin.

Currently, we need to clone restructuredText renderer manually. There
was also some brainsplit between readme file and the reality. It should
be easier now, by using either --recurse-submodules for git clone or
git submodule update --update.

.travis.yml

@@ -6,7 +6,7 @@ before_install:
   - sudo apt-get install python-pip -y
   - sudo pip install docutils pygments
 before_script:
-   - git clone https://github.com/gryf/jekyll-rst _plugins/jekyll-rst
+   - git submodule update --init
 install:
   - bundle install
 script: bundle exec jekyll build
@@ -18,5 +18,3 @@ deploy:
   target_branch: master
   on:
     branch: source
-
-