Simplify travis file

[chriz-zeller]

As a reference see:

https://docs.travis-ci.com/user/languages/haskell/ and https://github.com/ivanperez-keera/dunai/blob/develop/.travis.yml

That is, use the standard building image for Haskell ("ghc" variable, remove install section, etc.).

[chriz-zeller]

This travis file can be used as reference for the additionally packages that need to be installed before_install:

https://github.com/ivanperez-keera/SpaceInvaders/blob/develop/.travis.yml

Besides:

notifications:
email: true

is by default true and therefore can be deleted.

[Rizary]

have you considered this https://github.com/haskell-CI/haskell-ci for ci?

[ivanperez-keera]

I have not (but I may have looked at it, I don't remember).

[ivanperez-keera]

Closed by #59 .