lts update

[opyapeus]

Hello.
Thank you for providing this useful library!

But now, build failed with ghc-8.2.2.
Because of this, We cannot read docs at hackage (can at stackage).
https://hackage.haskell.org/package/sdl2-mixer-1.1.0/reports/

I've build it with ghc-8.4.3 (lts-12.4).
Then it works. (build os: os x 10.13.6)

I think it should be updated to new lts version.
Please check this pr. Thank you.

[opyapeus]

https://docs.travis-ci.com/user/languages/haskell/#specifying-haskell-compiler-versions

Travis doc says that how to specify ghc version.

ghc:
  - "7.10"
  - "7.8"
  - "7.6"
  - "8.4.1"

I tried 8.4.3 and 8.4.1 (example says), but not found version.

https://travis-ci.org/opyapeus/sdl2-mixer/builds/413732082
https://travis-ci.org/opyapeus/sdl2-mixer/builds/413744132

the error message seems to travis supports ghc version up to 8.0.2.

So, I’m pending.