Not compatible with PureScript 0.13.8

[thomashoneyman]

PureScript's 0.13.8 release changed the format of externs to use a binary encoding PR here.

If you try to use Zephyr on a project compiled with 0.13.8 you'll see an error like this:

zephyr: output/Affjax/externs.json: openBinaryFile: does not exist (No such file or directory)

There are no longer externs.json files. Instead, this would be a externs.cbor file in the same location in the output directory.

We'll need to update Zephyr to accommodate the new format.

(As a related note, there has been some discussion (1, 2) about moving Zephyr into the compiler, which would help with staying up-to-date with big changes in the compiler. Do you have any interest in this?)

[thomashoneyman]

I see that this was already addressed in #34 -- sorry for the duplicate!