openquake --version info should use UTC time

[Lars Butler]

This CI test failure: http://ci.openquake.org/job/openquake/86/testReport/junit/tests/VersionInfoTestCase/test_info_with_all_data_in_place/

The reason why this test failed is because the CI server is running in a different timezone (most of the developers are in Central European Time).

Using datetime.utcfromtimestamp instead of datetime.fromtimestamp should be an appropriate fix. See: https://github.com/gem/openquake/blob/master/openquake/utils/version.py#L65