yarn: Silence warning from a peer-dep in react-intl.

Running `yarn` gives the following warning:

  warning " > [email protected]" has unmet peer dependency "prop-types@^15.5.4".

(No such warning if `yarn` stops after step "Resolving packages" and
prints "Already up-to-date"; but if it has any actual work to do, it
always shows this warning in step "Linking dependencies".)

That's the only warning we now routinely get from `yarn`, so let's
get all the way to warning-free.

I believe the proper fix is that this shouldn't be a peer-dependency
in `react-intl` at all; reported that as formatjs/formatjs#1382 .
But as a workaround, adding a direct dependency silences the warning.

package.json

@@ -104,6 +104,7 @@
     "prettier": "^1.18.2",
     "prettier-eslint": "^9.0.0",
     "prettier-eslint-cli": "^5.0.0",
+    "prop-types": "^15.7.2",
     "react-native-cli": "^2.0.1",
     "redux-mock-store": "^1.5.1"
   },