Upgrade to React Native 0.56

Resolves zulip#2788

Upgrade dependencies and Flow version

.flowconfig

@@ -96,4 +96,4 @@ suppress_comment=\\(.\\|\n\\)*\\$FlowFixedInNextDeploy
 suppress_comment=\\(.\\|\n\\)*\\$FlowExpectedError
 
 [version]
-^0.67.0
+^0.75.0

package.json

@@ -50,9 +50,9 @@
     "lodash.unescape": "^4.0.1",
     "lodash.union": "^4.6.0",
     "lodash.uniqby": "^4.4.0",
-    "react": "16.3.2",
+    "react": "16.4.1",
     "react-intl": "^2.4.0",
-    "react-native": "^0.55.4",
+    "react-native": "0.56.1",
     "react-native-device-info": "^0.21.5",
     "rn-fetch-blob": "^0.10.13",
     "react-native-image-picker": "^0.26.10",
@@ -69,7 +69,7 @@
     "react-navigation": "^1.5.12",
     "react-navigation-redux-helpers": "^1.1.1",
     "react-redux": "^5.0.7",
-    "react-test-renderer": "16.3.2",
+    "react-test-renderer": "16.4.1",
     "redux": "^4.0.0",
     "redux-action-buffer": "^1.2.0",
     "redux-batched-actions": "^0.3.0",
@@ -87,9 +87,9 @@
     "@babel/core": "^7.1.2",
     "babel-core": "^6.26.3",
     "babel-eslint": "^8.2.3",
-    "babel-jest": "^22.4.4",
+    "babel-jest": "^23.4.0",
     "babel-plugin-transform-remove-console": "^6.9.4",
-    "babel-preset-react-native": "^4.0.0",
+    "babel-preset-react-native": "^5",
     "coveralls": "^3.0.1",
     "deep-freeze": "^0.0.1",
     "detox": "^7.3.7",
@@ -105,11 +105,11 @@
     "eslint-plugin-react": "^7.8.2",
     "eslint-plugin-react-native": "^3.2.1",
     "eslint-plugin-spellcheck": "0.0.6",
-    "flow-bin": "^0.67.0",
+    "flow-bin": "0.75",
     "flow-coverage-report": "^0.6.0",
     "flow-typed": "^2.4.0",
-    "jest": "^22.4.4",
-    "jest-cli": "^22.4.4",
+    "jest": "^23.4.0",
+    "jest-cli": "^23.4.0",
     "jest-react-native": "^18.0.0",
     "prettier": "^1.12.1",
     "prettier-eslint": "^8.8.1",