feat: move pretty-format from peer do deps (#39)

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### Summary

We could also make it an optional dependency and inline-require it inside debug (the only place we use it) but for now I decided to make it a regular dependency, because that's what it is, no point in making users install it directly.

### Test plan

green CI

README.md

@@ -57,7 +57,7 @@ Open a Terminal in your project's folder and run:
 yarn add --dev react-native-testing-library
 ```
 
-This library has a peerDependencies listing for `react-test-renderer`, `pretty-format` and, of course, `react`. Make sure to install them too!
+This library has a peerDependencies listing for `react-test-renderer` and, of course, `react`. Make sure to install them too!
 
 As you may have noticed, it's not tied to React Native at all – you can safely use it in your React components if you feel like not interacting directly with DOM.
 

package.json

@@ -23,7 +23,6 @@
     "flow-bin": "^0.83.0",
     "jest": "^23.6.0",
     "metro-react-native-babel-preset": "^0.48.0",
-    "pretty-format": "^23.6.0",
     "react": "16.6.0-alpha.8af6728",
     "react-native": "^0.57.3",
     "react-test-renderer": "16.6.0-alpha.8af6728",
@@ -31,8 +30,10 @@
     "strip-ansi": "^5.0.0",
     "typescript": "^3.1.1"
   },
+  "dependencies": {
+    "pretty-format": "^23.6.0"
+  },
   "peerDependencies": {
-    "pretty-format": ">=20.0.0",
     "react": ">=16.0.0",
     "react-test-renderer": ">= 16.0.0"
   },