Fix config path parsing with ts files

[JakeDawkins]

resolves #881

Previously, when initializing an ApolloConfig class, we were pulling the dirname properly off the file path of a .js config file, but we weren't handling the case of the config filepath being a .ts file at all (oops).

This change should fix the dirname parsing for all files that we support.

TODO:

• Update CHANGELOG.md* with your change (include reference to issue & this PR)
• Make sure all of the significant new logic is covered by tests
• Rebase your changes on master so that they can be merged easily
• Make sure all tests and linter rules pass

*Make sure changelog entries note which project(s) has been affected. See older entries for examples on what this looks like.