Fix date format for various locale

bashing generate a date header depend on the locale:

```
# Date (UTC):   2014年  2月 17日 月曜日 15:41:16 UTC
```
(https://github.com/kui/tumblr-gif/blob/6da77af6f97e8728f6e0b22e00e9b1d9e51527fb/bin/tumblr-gif#L5)

I fixed bashing which generate a date header:

```
# Date (UTC):   Mon, 17 Feb 2014 15:54:25 +0000
````
(https://github.com/kui/tumblr-gif/blob/master/bin/tumblr-gif#L5)