JacocoSourceReportFactory: fix filename passed to SourceReport

We need to use the path relative to the current working directory ('.'),
not only the canonical Java filename of <package>/class.java.

This fixes kt3k/coveralls-gradle-plugin #10.

Note that the approach is a little circuitous, and seems better handled by https://github.com/kt3k/coveralls-gradle-plugin/blob/master/src/main/groovy/org/kt3k/gradle/plugin/coveralls/domain/CoberturaSourceReportFactory.groovy#L73. However, the structure of the JacocoSourceReportFactory code is sufficiently different from the Cobertura code that it didn't seem worth rewriting the Jacoco code to mimic it.