Fix compatibility with TemplateHaskell from GHC 7.4

GHC 7.4 has updated the TemplateHaskell library, and it turns out that
the way we built the JSON instance implementation for showJSON was not
good (probably this is why GHC 6.12 was generating some warnings).

The patch changes the build of showJSON to be the same as readJSON,
which was working fine. As a bonus, this fixes both the 7.4 issue and
the 6.12 one.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: René Nussbaumer <rn@google.com>