Simplify Travis file with default Haskell image

[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.).

[ivanperez-keera]

Ok. So, just to clarify this issue, what you are saying is that it needs to do what it does, but more simply (using the simpler, standard image).

So, would what remains at this point is to make sure the examples are compiled, is that right?

[chriz-zeller]

The simplified script does not contain the flag for examples. I guess install and script should be modified in the simplified script to make sure that the examples are compiled.

Explanation:

With the recent simplification we are using the default install and script.
That is for install:

cabal install --only-dependencies --enable-tests

That is for script:

cabal configure --enable-tests && cabal build && cabal test

See also
https://docs.travis-ci.com/user/languages/haskell/

Both steps can be modified by adding install and/or script in .travis.yml while the rest of the simplification can remain as it is.