Convert milliseconds to microseconds before passing to Time::utc

e.g. given (from the CloudFormation api) a response including

<LastUpdatedTime>2014-07-02T13:40:04.141Z</LastUpdatedTime>

then the last_updated_time comes back with a seconds fractional part of .000141 (where it should be .141).

This is because the SDK currently takes the integer fractional part (141) - which is milliseconds, because the parser is hard-wired to three digits - and then passes it to Time::utc, which actually expects microseconds.

The bug wasn't spotted before because Time#to_s doesn't include fractional parts of a second.