Update to work with aeson 0.7

[nc6]

Version 0.7 of aeson drops instances from FromJSON ByteString (because a JSON string has particular semantics that a bytestring does not satisfy). This makes lots of sense, but breaks tabula which tries to serialise bytestrings.

The real solution to this would be to perfom some kind of sensible encoding (which may drop nonprintable characters) into a UTF-8 format stored as Text, for which FromJSON is still provided.

[jrandall]

This seems to occur when running emacs