lcov spec says the SF path should be an absolute path.

http://ltp.sourceforge.net/coverage/lcov/geninfo.1.php

However istanbul is generating a releative path.
gotwarlost/istanbul#104

refix can help fix this issue by prefixing with an absolute path.
prefix currently prepends ./ to the prefix which makes it a releative path and breaks it on windows.

fixes  mdasberg/grunt-karma-sonar#1