Generate date with UTC and 0 microseconds

The generated latest date needs to have a UTC timezone and must have
zero seconds, otherwise when it's formatted as a string it won't
be able to be validated by Backdrop.

Changes the output from:

    2014-12-12T00:00:01.12345

to:

    2014-12-12T00:00:01+00:00
Change regex match() to search()

re.match() only matches from the beginning of the line, but sometimes
we have matchers that rely on the caret to differentiate between
the numerator and denominator.

This commit changes to re.search(), which does not start looking
at the beginning of the string.
Merge pull request #393 from alphagov/bugfix-transformer-rate

Bugfix transformers