[Dev Deps] downgrade jest to v21, since we still support node v4

Author: ljharb

package.json

@@ -33,7 +33,7 @@
     "babel-cli": "^6.26.0",
     "babel-core": "^6.26.3",
     "babel-eslint": "^8.2.3",
-    "babel-jest": "^22.4.4",
+    "babel-jest": "^21.2.0",
     "babel-plugin-transform-es2015-template-literals": "^6.22.0",
     "babel-plugin-transform-flow-strip-types": "^6.22.0",
     "babel-plugin-transform-object-rest-spread": "^6.26.0",
@@ -44,10 +44,10 @@
     "eslint-config-airbnb-base": "^12.1.0",
     "eslint-plugin-flowtype": "^2.46.3",
     "eslint-plugin-import": "^2.12.0",
-    "expect": "^22.4.3",
+    "expect": "^21.2.1",
     "flow-bin": "^0.66.0",
     "in-publish": "^2.0.0",
-    "jest": "^22.4.4",
+    "jest": "^21.2.1",
     "jscodeshift": "^0.4.0",
     "minimist": "^1.2.0",
     "object.assign": "^4.1.0",
@@ -85,5 +85,11 @@
       "<rootDir>/__tests__/__util__"
     ],
     "testEnvironment": "node"
+  },
+  "greenkeeper": {
+    "ignore": [
+      "jest",
+      "babel-jest"
+    ]
   }
 }