[FLINK-13725][docs] use sassc for faster doc generation

Jekyll requires sass but can optionally also use a C-based implementation
provided by sassc. Although we do not use sass directly, there may be some
indirect use inside jekyll. It doesn't seem to hurt to upgrade here.

This closes apache#9443

docs/Gemfile

@@ -28,6 +28,7 @@ gem 'json', '2.2.0'
 gem 'jekyll-multiple-languages', '2.0.3'
 gem 'jekyll-paginate', '1.1.0'
 gem 'liquid-c', '4.0.0' # speed-up site generation
+gem 'sassc', '2.0.1' # speed-up site generation
 
 group :jekyll_plugins do
  gem 'hawkins'

docs/Gemfile.lock

@@ -55,6 +55,7 @@ GEM
  pathutil (0.16.2)
  forwardable-extended (~> 2.6)
  public_suffix (3.1.1)
+ rake (12.3.3)
  rb-fsevent (0.10.3)
  rb-inotify (0.10.0)
  ffi (~> 1.0)
@@ -67,6 +68,9 @@ GEM
  sass-listen (4.0.0)
  rb-fsevent (~> 0.9, >= 0.9.4)
  rb-inotify (~> 0.9, >= 0.9.7)
+ sassc (2.0.1)
+ ffi (~> 1.9)
+ rake
  sawyer (0.8.2)
  addressable (>= 2.3.5)
  faraday (> 0.8, < 2.0)
@@ -86,6 +90,7 @@ DEPENDENCIES
  json (= 2.2.0)
  liquid-c (= 4.0.0)
  octokit (= 4.14.0)
+ sassc (= 2.0.1)
  therubyracer (= 0.12.3)
 
 RUBY VERSION