Chores: upgrade Jest

Appears to be a bug where it gets the wrong `rootDir` in versions `20`
and up. Discovered a workaround that fixes the issue, though it appears
to be a bug.

jestjs/jest#3613 (comment)

config/jest.json

@@ -16,6 +16,7 @@
     "<rootDir>/vendor/assets/javascripts/",
     "<rootDir>/spec/javascript/"
   ],
+  "rootDir": "../",
   "setupTestFrameworkScriptFile": "<rootDir>/spec/javascript/test_helper.js",
   "testRegex": "/spec/javascript/.*_spec.js(x)?$",
   "testURL": "http://test.host"

package.json

@@ -49,15 +49,15 @@
     "jest locked at 19 because of https://github.com/facebook/jest/issues/3499"
   ],
   "devDependencies": {
-    "babel-jest": "^20.0.3",
+    "babel-jest": "^21.2.0",
     "enzyme": "^3.0.0",
     "enzyme-adapter-react-16": "^1.0.1",
     "eslint": "^4.5.0",
     "eslint-plugin-import": "^2.7.0",
     "eslint-plugin-jest": "^21.2.0",
     "eslint-plugin-jsx-a11y": "^6.0.2",
     "eslint-plugin-react": "^7.3.0",
-    "jest": "^19.0.2",
+    "jest": "^21.2.1",
     "jest-enzyme": "^4.0.0",
     "jsdom": "^11.2.0",
     "react-test-renderer": "^16.0.0",