Fix for 'no writecb in Transform' error.
Fix for 'no writecb in Transform' error.

The clue came from the discussion here:
https://github.com/rvagg/through2/issues/1

Dakuan/gulp-jest,M6Web/gulp-jest,ncuillery/gulp-jest,daspecster/gulp-jest-iojs,alansouzati/gulp-jest,abiee/gulp-jest,james-hart/gulp-jest