Allow to parse file through relative path

#47 breaks a case that the 'Source' line in `.gcov` point to a relative path within the project, where the base directory of this relative path is the current directory instead of the project root.

See https://travis-ci.org/eddyxu/vobla/jobs/25918695

@sjaeckel  Could you take a look of this PR to verify whether it breaks your case?

Thanks!