Updated wpxmlrpc to 0.4.3

Fixes an issue with date encoding when the current locale uses a
non-gregorian calendar

Fixes #2139

See https://github.com/wordpress-mobile/wpxmlrpc/issues/15
Merge pull request #2572 from wordpress-mobile/issue/2139-gregorian-dates

#2139 Updated wpxmlrpc to 0.4.3