Update xunit.js

Changed testsuite Attribute "duration" to "time".
Jenkins does not parse the generated xunit Files otherways.

See also:
https://groups.google.com/d/msg/casperjs/n1tbi0K9Tnk/MA4SO_4SImUJ

The XSD Schema:
http://windyroad.com.au/dl/Open%20Source/JUnit.xsd