fix(configuration): Remove unnecessary options
fix(configuration): Remove unnecessary options

The roots and testPathDirs attributes of the configuration passed to Jest may both be left undefined.  

This has the added advantage of continuing to work once testPathDirs has been deprecated in Jest 19.0.0.  See facebook/jest#2776
mthmulders/stryker-jest-runner,mthmulders/stryker-jest-runner,mthmulders/stryker-jest-runner