Use kind-of >=6.0.3 everywhere (#590)
* Use kind-of >=6.0.3 everywhere This repo's dependencies currently use [older versions](https://github.com/paritytech/fether/blob/3c036307/yarn.lock#L10218) of kind-of which have a possibly exploitable vulnerability ([original issue](https://github.com/jonschlinkert/kind-of/issues/30), [PR](https://github.com/jonschlinkert/kind-of/pull/31)). * Update yarn.lock