Update default license to ISC

[npmconf now provides ISC as the default init.license](https://github.com/npm/npmconf/commit/160aa090f16159e0639a1a26f4115ed0ccca90e4#diff-093e169a50df5b94ced78237e58e4cf5R247), init-package-json could probably follow suit, in the case that init.license becomes unset?