"en_US" to "en_US_POSIX"

[ShingoFukuyama]

JTBS

https://developer.apple.com/library/ios/qa/qa1480/_index.html

In most cases the best locale to choose is "en_US_POSIX", a locale that's specifically designed to yield US English results regardless of both user and system preferences. "en_US_POSIX" is also invariant in time (if the US, at some point in the future, changes the way it formats dates, "en_US" will change to reflect the new behaviour, but "en_US_POSIX" will not), and between machines ("en_US_POSIX" works the same on iOS as it does on OS X, and as it it does on other platforms).

[ccgus]

I'll consider changing this for 3.0. I can't make a change like this in the middle of a release.