Update of travis file (add newer ghc versions)

[chriz-zeller]

The travis file could be updated.

• Instead of 8.0.1 it should be 8.2.2.
• 8.4.4 and 8.6.1 should be added.
• The condition in line 40 needs to be adjusted. It should be the highest included ghc version.

Hopefully all of them compile without problems.

[belushkin]

Let me take it

[ivanperez-keera]

Is 8.6.2 available? 8.6.1 was unstable.

[chriz-zeller]

@ivanperez-keera I searched for the newest versions here: https://www.haskell.org/ghc/
There I couldn't found 8.6.2.

I couldn't found the source that says that 8.6.1 is unstable in general. Do you refer to this source: https://mail.haskell.org/pipermail/haskell-cafe/2018-September/130015.html ?

[chriz-zeller]

Here are the notes from an off line discussion:

8.6.1 seems to be broken as stated here:
https://www.reddit.com/r/haskell/comments/9ksch6/ghc_861_is_badly_broken/

8.6.2 seems to be available, but was not announced yet:
https://launchpad.net/%7Ehvr/+archive/ubuntu/ghc/+index?batch=75&memo=75&start=75

That is, the status of 8.6.2 is unclear.

We will try to include a 8.6 version for being informed about the status, but we will tell travis that it is okay to fail on this branch using something like this:
https://docs.travis-ci.com/user/customizing-the-build/#rows-that-are-allowed-to-fail

[chriz-zeller]

Closed by #81 .

[chriz-zeller]

Version 8.6.2 fails due to the following reason:

$ export TRAVIS_HASKELL_VERSION=$(travis_ghc_find '8.6.2')
$ export PATH=${TRAVIS_GHC_ROOT}/${TRAVIS_HASKELL_VERSION}/bin:${PATH}
$ export PATH=/opt/ghc/${TRAVIS_HASKELL_VERSION}/bin:${PATH}

Therefore, it is best to go back to 8.6.1, which is as stated above broken, but at least can be found for testing.