chore(babylon): Update babylon to 7.0.0-beta.20

beta 21+ breaks react-docgen because of babel/babylon#713

Author: danez

package.json

@@ -34,7 +34,7 @@
   "dependencies": {
     "async": "^2.1.4",
     "babel-runtime": "^6.9.2",
-    "babylon": "v7.0.0-beta.17",
+    "babylon": "v7.0.0-beta.20",
     "commander": "^2.9.0",
     "doctrine": "^2.0.0",
     "node-dir": "^0.1.10",

yarn.lock

@@ -752,9 +752,9 @@ babylon@^6.17.0, babylon@^6.17.2, babylon@^6.17.4:
   version "6.17.4"
   resolved "https://registry.yarnpkg.com/babylon/-/babylon-6.17.4.tgz#3e8b7402b88d22c3423e137a1577883b15ff869a"
 
-[email protected].17:
-  version "7.0.0-beta.17"
-  resolved "https://registry.yarnpkg.com/babylon/-/babylon-7.0.0-beta.17.tgz#2aad4d6764f409dfb3ac216d855dc93d70d37911"
+[email protected].20:
+  version "7.0.0-beta.20"
+  resolved "https://registry.yarnpkg.com/babylon/-/babylon-7.0.0-beta.20.tgz#7dbc70cc88de13334066fe5200e0efaa30c0490e"
 
 balanced-match@^1.0.0:
   version "1.0.0"