Follow TH changes in GHC 7.9. Fixes #410.

Pred data type is now an alias to Type.

Note that you'll at least need https://github.com/bos/aeson/commit/19682fe0a5136eb84e674bda1ab76880bb3281e1 which doesn't seem to be on Hackage yet.

With `cabal configure --enable-tests -f-test-doctests && cabal test` I get 4 out of 4 test-suites passing on GHC 7.6.3 and GHC 7.9.20140308.