chore(package): update @types/json-schema to version 7.0.4

Closes #20

Author: greenkeeper[bot]

package.json

@@ -23,7 +23,7 @@
   "homepage": "https://github.com/shapeshiftjs/shapeshift-core#readme",
   "devDependencies": {
     "@types/jest": "^23.0.0",
-    "@types/json-schema": "^6.0.0",
+    "@types/json-schema": "^7.0.4",
     "coveralls": "^3.0.0",
     "jest": "^22.0.3",
     "jest-cli": "^22.0.3",