update yeoman-generator to 0.21 because of RegExp DoS issue

Summary:
Installing react-native gives a warning because of outdated version of yeoman-generator. The warning is ```npm WARN deprecated minimatch@2.0.10: Please update to minimatch 3.0.2 or higher to avoid a RegExp DoS issue```.
This MR will update the yeoman-generator version to 0.21.0 to resolve this warning.
Closes https://github.com/facebook/react-native/pull/8859

Reviewed By: matryoshcow

Differential Revision: D3580087

Pulled By: bestander

fbshipit-source-id: 4daddd804679ab8e95e59cf0d0005f87d2f48e1c

package.json

@@ -187,8 +187,8 @@
     "xcode": "^0.8.2",
     "xmldoc": "^0.4.0",
     "yargs": "^3.24.0",
-    "yeoman-environment": "~1.2.7",
+    "yeoman-environment": "1.5.3",
-    "yeoman-generator": "^0.20.3",
+    "yeoman-generator": "0.21.2",
     "mime-types": "2.1.11",
     "whatwg-fetch": "^1.0.0"
   },