Switch msgpack to messagepack

I'd like to replace msgpack because fluent-logger doesn't properly with it (I'm using ghc-7.8.3) and msgpack.org recommends rodrigosetti's messagepack library.
- Replace msgpack with messagepack 
- Remove ghc-7.4 and Haskell Platform support from .travis.yml because messagepack requires attoparsec-0.12
- There is some incompatibility due to the change of library