[docs] require at least Ruby 1.9.0 for building

The json module is bundled with Ruby from 1.9.0 and upwards. This
cures dependency problems with different versions of Ruby.

docs/Gemfile

@@ -18,9 +18,10 @@
 
 source 'https://rubygems.org'
 
+ruby '>=1.9.0'
+
 # Dependencies required to build the Flink docs
 gem 'jekyll', '2.5.3'
-gem 'json', '2.0.1'
 gem 'kramdown', '1.10.0'
 gem 'pygments.rb', '0.6.3'
 gem 'therubyracer', '0.12.2'