NEWS: note --with-jemalloc option [ci skip]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47631 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
* 2014-09-19

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47632 b2dd03c8-39d4-4d8f-98ff-823fe69b080e