Adjust json dependency to match ruby 1.9.2 and try to load a newer json gem, if available

Author: drbrain

Rakefile

@@ -40,7 +40,7 @@ Depending on your version of ruby, you may need to install ruby rdoc/ri data:
   self.clean_globs += PARSER_FILES
 
   require_ruby_version '>= 1.8.7'
-  extra_deps     << ['json',     '~> 1.5']
+  extra_deps     << ['json',     '~> 1.4']
   extra_dev_deps << ['racc',     '~> 1.4']
   extra_dev_deps << ['minitest', '~> 2']
   extra_dev_deps << ['ZenTest',  '~> 4']

lib/rdoc/text.rb

@@ -8,6 +8,11 @@
 ##
 # For RDoc::Text#snippet
 
+begin
+  gem 'json'
+rescue Gem::LoadError
+end
+
 require 'json'
 
 ##