Fix ghc-parser on GHC 7.8.3

In GHC 7.8.3, GHC's parser changed a bit, causing ghc-parser to fail to
build.  I fixed it by following the instructions on issue #268.

Note: I was unable to get the test suite to pass, but this is the first time I've ever tried running the test suite, so that might not be related to GHC 7.8.3 and might instead just be me not knowing how to run the test suite properly. Someone who knows what they're doing, please test :)