Fix CI for node < 8

Summary:
Thanks alloy for noticing that 0d24a72 broke CI.  The reason is that node < 8 does not support trailing commas for function calls which prettier adds in our configuration. This should fix it.
Closes #2373

Differential Revision: D7311796

Pulled By: kassens

fbshipit-source-id: fcc7a172917d398e8022d3bb6f5f5f1a90f20e0d

scripts/jest/preprocessor.js

@@ -61,9 +61,11 @@ module.exports = {
   getCacheKey: createCacheKeyFunction([
     __filename,
     testSchemaPath,
+    // We cannot have trailing commas in this file for node < 8
+    // prettier-ignore
     path.join(
       path.dirname(require.resolve('babel-preset-fbjs')),
-      'package.json',
+      'package.json'
     ),
     path.join(__dirname, '..', 'getBabelOptions.js'),
   ]),