Allow working with GHC HEAD

The current version of JSON breaks with GHC head since now the applicative operators are exported from the Prelude. I fixed that by renaming the local operators slightly so they don't conflict. I had originally wanted to use CPP to get it working nicely, but that requires demanding a higher version of parsec and some careful GHC version switching. If anyone ever gives the package a thorough cleanup they probably want to do that, but this keeps it working for now and isn't too horrible.

At the same time I also added a .travis.yml file to test it with Travis on lots of GHC versions - see [here](https://travis-ci.org/ndmitchell/json) to see them all passing. I also added a source-repo link, since I didn't find this repo the first time round.