Have make_travis_yml_2.hs test that Haddocks build

[seagreen]

"Check for 100% Haddock coverage" is currently listed in the Additional Checks section, but a smaller check that everyone would probably want would be just making sure the Haddocks build.

[hvr]

That's a good idea; I'm just waiting for cabal new-haddock to become a bit more reliable. But you could experiment yourself by trying to insert an invocation of cabal new-haddock in the generated .travis.yml scripts and report back how well it works for you. Maybe it's already good enough so that we can enable it for everybody.

[seagreen]

I tested out cabal new-haddock and it works great. I've got it added on hjsonschema's master now so if any problems do come up I'll let you know.

[hvr]

note to myself: need to workaround haskell/cabal#4506

[phadej]

new--haddock is in the v2 script