Add a travis check to validate nls/strings.js and nls/urls.js

[jasonsanjose]

I noticed src/nls/strings.js and src/nls/urls.js were out of sync when reviewing #4741, see #5038 for the fix.

It would be nice if we could fail the travis tests when

• the list of languages in src/nls/strings.js did not match the subdirectories of src/nls
• the list of getting started languages in src/nls/urls.js did not match the subdirectories of samples and languages that specify custom URLs for third party licenses.

[njx]

To @redmunds, low priority. Probably just need to write a bit of custom node code and have it run by the grunt test task.