Remove deprecated has_rdoc from gemspec
diff --git a/lib/rb/thrift.gemspec b/lib/rb/thrift.gemspec
index 86d2189..1977ec4 100644
--- a/lib/rb/thrift.gemspec
+++ b/lib/rb/thrift.gemspec
@@ -12,7 +12,6 @@
   s.license     = 'Apache-2.0'
   s.extensions  = ['ext/extconf.rb']
 
-  s.has_rdoc      = true
   s.rdoc_options  = %w[--line-numbers --inline-source --title Thrift --main README]
 
   s.rubyforge_project = 'thrift'