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/3c0363079e1148ae0dc0595523905f9bee7b9a84/yarn.lock#L10218) of kind-of which have a possibly exploitable vulnerability ([original issue](jonschlinkert/kind-of#30), [PR](jonschlinkert/kind-of#31)).

* Update yarn.lock

package.json

@@ -77,5 +77,8 @@
     "semver": "^5.6.0",
     "ts-node": "^8.0.3",
     "typescript": "^3.3.4000"
+  },
+  "resolutions": {
+    "kind-of": ">=6.0.3"
   }
 }

yarn.lock

@@ -9126,7 +9126,7 @@ is-binary-path@^1.0.0:
   dependencies:
     binary-extensions "^1.0.0"
 
-is-buffer@^1.0.2, is-buffer@^1.1.4, is-buffer@^1.1.5:
+is-buffer@^1.1.4:
   version "1.1.6"
   resolved "https://registry.yarnpkg.com/is-buffer/-/is-buffer-1.1.6.tgz#efaa2ea9daa0d7ab2ea13a97b2b8ad51fefbe8be"
   integrity sha512-NcdALwpXkTm5Zvvbk7owOUSvVvBKDgKP5/ewfXEznmQFfs4ZRmanOeKBTjRVjka3QFoN6XJ+9F3USqfHqTaU5w==
@@ -10215,36 +10215,10 @@ killable@^1.0.1:
   resolved "https://registry.yarnpkg.com/killable/-/killable-1.0.1.tgz#4c8ce441187a061c7474fb87ca08e2a638194892"
   integrity sha512-LzqtLKlUwirEUyl/nicirVmNiPvYs7l5n8wOPP7fyJVpUPkvCnW/vuiXGpylGUlnPDnB7311rARzAt3Mhswpjg==
 
-kind-of@^2.0.1:
-  version "2.0.1"
-  resolved "https://registry.yarnpkg.com/kind-of/-/kind-of-2.0.1.tgz#018ec7a4ce7e3a86cb9141be519d24c8faa981b5"
-  integrity sha1-AY7HpM5+OobLkUG+UZ0kyPqpgbU=
-  dependencies:
-    is-buffer "^1.0.2"
-
-kind-of@^3.0.2, kind-of@^3.0.3, kind-of@^3.2.0:
-  version "3.2.2"
-  resolved "https://registry.yarnpkg.com/kind-of/-/kind-of-3.2.2.tgz#31ea21a734bab9bbb0f32466d893aea51e4a3c64"
-  integrity sha1-MeohpzS6ubuw8yRm2JOupR5KPGQ=
-  dependencies:
-    is-buffer "^1.1.5"
-
-kind-of@^4.0.0:
-  version "4.0.0"
-  resolved "https://registry.yarnpkg.com/kind-of/-/kind-of-4.0.0.tgz#20813df3d712928b207378691a45066fae72dd57"
-  integrity sha1-IIE989cSkosgc3hpGkUGb65y3Vc=
-  dependencies:
-    is-buffer "^1.1.5"
-
-kind-of@^5.0.0:
-  version "5.1.0"
-  resolved "https://registry.yarnpkg.com/kind-of/-/kind-of-5.1.0.tgz#729c91e2d857b7a419a1f9aa65685c4c33f5845d"
-  integrity sha512-NGEErnH6F2vUuXDh+OlbcKW7/wOcfdRHaZ7VWtqCztfHri/++YKmP51OdWeGPuqCOba6kk2OTe5d02VmTB80Pw==
-
-kind-of@^6.0.0, kind-of@^6.0.2:
-  version "6.0.2"
-  resolved "https://registry.yarnpkg.com/kind-of/-/kind-of-6.0.2.tgz#01146b36a6218e64e58f3a8d66de5d7fc6f6d051"
-  integrity sha512-s5kLOcnH0XqDO+FvuaLX8DDjZ18CGFk7VygH40QoKPUQhW4e2rvM0rwUq0t8IQDOwYSeLK01U90OjzBTme2QqA==
+kind-of@>=6.0.3, kind-of@^2.0.1, kind-of@^3.0.2, kind-of@^3.0.3, kind-of@^3.2.0, kind-of@^4.0.0, kind-of@^5.0.0, kind-of@^6.0.0, kind-of@^6.0.2:
+  version "6.0.3"
+  resolved "https://registry.yarnpkg.com/kind-of/-/kind-of-6.0.3.tgz#07c05034a6c349fa06e24fa35aa76db4580ce4dd"
+  integrity sha512-dcS1ul+9tmeD95T+x28/ehLgd9mENa3LsvDTtzm3vyBEO7RPptvAD+t44WVXaUjTBRcrpFeFlC8WCruUR456hw==
 
 kleur@^3.0.3:
   version "3.0.3"