Remove warning caused by deprecated 'transform' section in Jest config (

#253)

```
ts-jest[main] (WARN) Replace any occurrences of "ts-jest/dist/preprocessor.js" or  "<rootDir>/node_modules/ts-jest/preprocessor.js" in the 'transform' section of your Jest config with just "ts-jest".
```

package.json

@@ -48,7 +48,7 @@
   },
   "jest": {
     "transform": {
-      ".(ts|tsx)": "<rootDir>/node_modules/ts-jest/preprocessor.js"
+      ".(ts|tsx)": "ts-jest"
     },
     "testEnvironment": "node",
     "testRegex": "(/__tests__/.*|\\.(test|spec))\\.(ts|tsx|js)$",