add jest types so tests will compile

Author: cellog

package-lock.json

@@ -63,6 +63,7 @@
     "@babel/preset-flow": "^7.0.0",
     "@babel/preset-typescript": "^7.3.3",
     "@babel/register": "^7.4.4",
+    "@types/jest": "^24.0.17",
     "@typescript-eslint/eslint-plugin": "^1.13.0",
     "@typescript-eslint/parser": "^1.13.0",
     "babel-core": "^7.0.0-bridge.0",