chore(package): update babel-plugin-transform-flow-strip-types to version 6.21.0 (#3250)

https://greenkeeper.io/

Author: greenkeeperio-bot

package.json

@@ -48,7 +48,7 @@
     "babel-core": "6.20.0",
     "babel-eslint": "^7.1.1",
     "babel-plugin-syntax-flow": "6.13.0",
-    "babel-plugin-transform-flow-strip-types": "6.18.0",
+    "babel-plugin-transform-flow-strip-types": "6.21.0",
     "babel-preset-es2015": "6.14.0",
     "babel-preset-stage-0": "6.5.0",
     "babel-register": "6.18.0",