deps: Use our fork of react-native.

Trying this as a workaround for
jestjs/jest#10221; see
https://github.com/chrisbobbe/react-native/tree/pr-0.61.5-zulip for
the contents.

If we actually use this, we shouldn't use the fork associated with
my GitHub account, but rather zulip/react-native, as we discuss in
our doc:
https://github.com/zulip/zulip-mobile/blob/2c1caa2a2aa5e574fca3acfb4d4fda0ca2adde5c/docs/architecture/our-rn.md

Currently breaks `tools/test android`; looks like we have a doc that
may help us through that, at
https://github.com/zulip/zulip-mobile/blob/master/docs/howto/rn-from-git.md.
The "Android" section links to an upstream doc that looks
interesting for this.

package.json

@@ -58,7 +58,7 @@
     "lodash.uniqby": "^4.4.0",
     "react": "16.9.0",
     "react-intl": "^2.4.0",
-    "react-native": "0.61.5",
+    "react-native": "chrisbobbe/react-native#pr-0.61.5-zulip",
     "react-native-device-info": "^0.21.5",
     "react-native-document-picker": "^3.2.4",
     "react-native-image-picker": "^0.26.10",

yarn.lock

@@ -8679,10 +8679,9 @@ react-native-webview@~10.0.0:
     escape-string-regexp "2.0.0"
     invariant "2.2.4"
 
-react-native@0.61.5:
+react-native@chrisbobbe/react-native#pr-0.61.5-zulip:
   version "0.61.5"
-  resolved "https://registry.yarnpkg.com/react-native/-/react-native-0.61.5.tgz#6e21acb56cbd75a3baeb1f70201a66f42600bba8"
-  integrity sha512-MXqE3NoGO0T3dUKIKkIppijBhRRMpfN6ANbhMXHDuyfA+fSilRWgCwYgR/YNCC7ntECoJYikKaNTUBB0DeQy6Q==
+  resolved "https://codeload.github.com/chrisbobbe/react-native/tar.gz/afd25ddd1319036a02e632e35050bffa42246849"
   dependencies:
     "@babel/runtime" "^7.0.0"
     "@react-native-community/cli" "^3.0.0"