add rbenv rehash

if you are using rbenv, you should run `rbenv rehash` after gem installed
Update promise-overview.adoc
Update promise-overview.adoc
Update promise-overview.adoc
add Ch2_HowToWrite/promise-and-array.adoc
merge
Update promise-overview.adoc
Update what-is-promise.adoc
Merge pull request #5 from kaku87/master

overview完成
LIBIN：先merge，最后需要统一修改术语“非同期”->“异步”
Merge branch 'master' into gh-pages
update promise overview