disabled snappy for GHC < 7.7

disabled snappy for GHC < 7.7, because ghc generates linker errors in
combination with tamplate haskell in our scotty server:

$ cabal build
...
Loading package ghc-prim ... linking ... done.
Loading package integer-gmp ... linking ... done.
Loading package base ... linking ... done.
Loading package array-0.4.0.1 ... linking ... done.
Loading package deepseq-1.3.0.1 ... linking ... done.
Loading package bytestring-0.10.0.2 ... linking ... done.
Loading package snappy-0.2.0.2 ... linking ... ghc:
/home/privat/.cabal/lib/snappy-0.2.0.2/ghc-7.6.3/HSsnappy-0.2.0.2.o:
unknown symbol `_ZTV8BSSource'
ghc: unable to load package `snappy-0.2.0.2'

AFAIK this is fixed in 7.7