Enable Travis CI

[maoe]

This PR enables Travis CI with various GHC versions including the latest one.

• Currently the write-merge test fails, which is a known issue (#14)
  • We may or may not turn off this test until #14 is resolved.
• I dropped support for ancient GHC versions
  • 6.12.3 isn't supported by hvr/multi-ghc-travis
  • 7.4.1 fails to build the latest containers due to Safe Haskell errors. I didn't bother fixing this because I don't think we need to support that old GHC.

[Mikolaj]

Splendid! LGTM.

[maoe]

I think it's better to disable write-merge for now because otherwise we have to check every travis build log to confirm nothing is actually broken.

[maoe]

Travis now skips write-merge: https://travis-ci.org/maoe/ghc-events/builds/200771369

[maoe]

Reworded the last commit message. s/is/are/

[kvelicka]

Only posted an insignificant question on one of the changes. Looks good!

@Mikolaj: shall I merge it or do we want someone else's opinion as well?

[Mikolaj]

IMHO, if @simonmar doesn't object, let's merge. Great job, BTW.

[kvelicka]

Right, pending on his call then.

[maoe]

Any objections?

[Mikolaj]

I think you can safely assume there are none. :)

[maoe]

Okay, could we merge then?

[Mikolaj]

O, you didn't have repo write access. My bad, I can actually add you. Done.

[maoe]

Great, thank you! I'm going to merge this then.

[maoe]

@Mikolaj Could you please enable Travis builds? I see ghc-events on https://travis-ci.org/profile/haskell but it's grayed out so I cannot enable it.

[Mikolaj]

Right. Supposedly it should work now.

[maoe]

Thanks!

[Mikolaj]

You are welcome.