Fix failing async test

[maxdeviant]

Currently the tests for the async functionality (added in #5) are failing, as noted in this comment:

Also, as you might have noticed, the test for the async way was failing. I've wrapped the stream in a promise that will resolve with the completed string when it's ready. But still the encryption test passes and decryption fails. I think the encryption is passing only due to the test state object being really small and it manages to resolve before expect() starts working.

These should be fixed.

[sibelius]

is this still a thing?

[maxdeviant]

@sibelius Yes, this test is still commented out because it's failing.

I don't think it's the test that's bad but rather we need to take a look at how the async functionality is implemented.

[maxdeviant]

Asynchronous support has been removed in #47.