change moment.lang() to moment.locale()

As for moment ^2.8.0, the moment.lang() method is deprecated, so we use locale() method instead, note that the localeData() method refered in the deprecation message is not chainable, please seehttps://github.com/icambron/moment/commit/a45c1290b57eb9d7953f8bd71ab98736b943b9bc
Merge pull request #822 from JLHwung/master

change moment.lang() to moment.locale()